Postcodes in the United Kingdom

Map of postcode areas in the United Kingdom and Crown dependencies, with links to each postcode area

Postal codes used in the United Kingdom, British Overseas Territories and Crown dependencies are known as postcodes (originally, postal codes). They are alphanumeric and were adopted nationally between 11 October 1959 and 1974, having been devised by the General Post Office (Royal Mail).

The system was designed to aid in sorting mail for delivery. It uses alphanumeric codes to designate geographic areas. A full postcode, identifies a group of addresses (typically around 10) or a major delivery point. It consists of an outward code and inward code. The outward code indicates the area and district, while the inward code specifies the sector and delivery point.

The initial postcode system evolved from named postal districts introduced in London and other large cities from 1857. Districts in London were then subdivided in 1917, with each allocated a distinct number, which extended to other cities by 1934. The territory of the UK is broken down into 121 postcode areas. Each postcode area contains multiple post towns and districts. Postcode areas are mnemonically named after the area's major post town (such as TR for TRuro) although some are named after smaller towns or regional areas.

Postcodes have since been additionally used in various applications. Postcodes help calculate insurance premiums, designate destinations in route planning software, and serve as aggregation units in census enumeration. The Postcode Address File (PAF) database stores and updates the boundaries and address data for around 29 million addresses, ensuring accurate delivery and extensive utility beyond postal services.

Overview

Structure

A full postcode is known as a "postcode unit" and designates an area with several addresses or a single major delivery point. The structure of a postcode is two alphanumeric codes that show, first, the post town and, second, a small group of addresses in that post town. The first alphanumeric code (the outward code or outcode) has between two and four characters and the second (the inward code or incode) always has three characters. The outcode indicates the postcode area and postcode district. It consists of one or two letters, followed by one digit, two digits, or one digit and one letter. This is followed by a space and then the incode which indicates the postcode sector and delivery point (usually a group of around 10-15 addresses, but can vary). The incode (always three characters), starts with a number (denoting a sector within the district), and ends with two letters (denoting delivery points which are allocated to streets, sides of a street or individual properties).

Postcode area names

Postcode areas are usually, but not always, named after a major town or city – such as B for Birmingham. Some are named after a smaller town (e.g. Southall postcode area is UB after Uxbridge) or a combination of towns (e.g. SM appears to be named after Sutton and Morden). A small number are regional – such as HS for the Outer Hebrides, FY for Fylde (the region around Blackpool) and ME for the Medway conurbation, with Rochester as its main post town. In the case of London (a Post Town), there is not a single "London" postcode area (such as "LO"); rather there are eight (N, E, EC, SE, SW, W, WC and NW) reflecting the preceding system for coding London based on compass points. In the case of Northern Ireland, the entire province has a single postcode area BT (named for Belfast). The mnemonic features various combinations - most commonly, first two letters (CH for CHester); first and last letters (BH for BournemoutH); first and key syllable letters (IV for InVerness). Postcodes generally do not align with historical county or local authority boundaries, and can also cross national boundaries (e.g. the CH and TD postcode areas).

Postcode districts and numbering

Each postcode area contains a number of post towns and postcode districts.

As a general rule, the central part of the town/city the postcode area is named after will have the number 1 e.g. B1 (central Birmingham) – but there a limited number of postcode areas that start 0 or 10, e.g. SL0 and AB10. Croydon uniquely has no CR1 despite having CR0 and CR2-CR10 (an unintended outcome from the initial pilot of three letter postcode areas, where "CRO" for Croydon was transferred to the new format and became CR0, rather than CR1). Large post towns are generally numbered from the centre outward such that outlying parts have higher numbered districts. In most post towns, the postcode "aa1 1AA" was allocated to the crown or principal post office. These are increasingly defunct, as post offices have closed or moved.

Alternatively, but less commonly, post towns and postcode districts within the area may be numbered according to a different pattern -

• geographical direction (e.g. the Outer Hebrides postcode area HS, where the districts are numbered from north to south)
• alphabetical order of district sub-offices, which is used only in London postcode areas (e.g. E2 is Bethnal Green, E3 is Bow, E4 is Chingford etc., which results in intuitive anomalies such as SE1 and SE2 being far from neighbours, with SE1 being large part of Central London south of the Thames (Waterloo and the borough of Southwark) and SE2 covering Abbey Wood at the far eastern end of the Elizabeth Line.
• clusters particularly if the postcode area encompasses several major towns or cities (e.g. S postcode area with Sheffield S1-S36, Chesterfield S40-S49, Rotherham S60-S65, Barnsley S70-S75 and Worksop S80-S81)
• earlier districts where preceding postal districts have been adopted or translated into the current system (such as Glasgow's "compass points") - see Earlier Postal Districts.

Numbering of postcode districts is normally consecutive, starting from "aa1" (e.g. Halifax which has seven districts numbered consecutively HX1 to HX7), but this is not universal. Non-consecutive numbering can arise when geographic realities are faced and consecutive numbering would be potentially misleading or unhelpful, or when new districts are created or added/removed from the postcode area.

• geographic reality: particularly in larger postcode areas, the scale of the districts prevents consecutive numbering e.g. NE postcode area's NE50-PH60 are not allocated due to the substantial distance between NE49 (Haltwhistle) and the next district to be numbered (Morpeth, which was numbered NE61 rather than NE50), recognising the geographic separation of districts in the west (up to NE49) from those to the north (NE61 upwards).
• new postcode districts: new postcode districts are occasionally created, usually due to increased demand for addresses following housing/business development and exhaustion of available postcodes for the existing district. There appear to be no clear rules for numbering new districts. If no sequential numbers are available, then new non-consecutive numbers are allocated. For example, in the CV postcode area, the CV47 district was formed in 1999 from parts of the CV23 and CV33 districts, where the highest numbered district then allocated was CV37.
• transfer of postcode districts: postcodes districts can be transferred for operational reasons e.g. PH49 and PH50 postcode districts which resulted from the transfer and recoding of PA39 and PA40 districts (PH39 and PH40 were already allocated although PH45-PH48 were not allocated). This also occurred with the creation of the HS postcode area, the only new geographic postcode area created since 1973, from the previous PA80 to PA87 districts.

London uniquely uses letters for subdivisions of some of its postcode districts (E1, N1, W1, WC1, WC2, EC1-EC4, NW1, SE1, SW1). These are effectively postcode districts in their own right, and could have been given double digit numbers (e.g. for WC in the range WC10-WC19 and WC20-WC29). In doing so, the districts would have been geographically extremely small, and may have encountered resistance from residents who feared some loss of status if re-coded from WC1 to, say, WC15.

Accordingly, many postcode districts are not physically contiguous, despite the inference from their numbering. Likewise, the centrality of a postcode district within a postcode area cannot be reliably inferred from the postcode alone. See postcode area.

Postcode use

Postcodes have been adopted for a wide range of purposes in addition to aiding the sorting of mail: for calculating insurance premiums, designating destinations in route planning software and as the lowest level of aggregation in census enumeration. The boundaries of each postcode unit and within these the full address data of currently about 29 million addresses (delivery points) are stored, maintained and periodically updated in the Postcode Address File database.

Theoretically, deliveries can reach their destination using the house number (or name if the house has no number) and postcode alone; however, this is against Royal Mail guidelines, which request the use of a full address.

History

Earlier postal districts

London

Main article: London postal district

The London post town covers 40% of Greater London. On inception (in 1857/8), it was divided into ten postal districts: EC (East Central), WC (West Central), N, NE, E, SE, S, SW, W, and NW. The S and NE sectors were later abolished. In 1917, as a wartime measure to improve efficiency, each postal district was subdivided into sub-districts each identified by a number; the area served directly by the district head office was allocated the number 1; the other numbers were allocated alphabetically by delivery office, e.g. N2 East Finchley delivery office, N3 Finchley delivery office, N4 Finsbury Park delivery office etc. Since then these sub-districts have changed little.

Some older road signs in Hackney still show the North East (NE) sector/district.

Other large cities and towns

Street name signs on Birdbrook Road, Great Barr, Birmingham, showing old "Birmingham 22" postal district (top) and modern "B44" postcode.

Following the successful introduction of postal districts in London, the system was extended to other large towns and cities. Liverpool was divided into Eastern, Northern, Southern and Western districts in 1864/65, and Manchester and Salford into eight numbered districts in 1867/68.

In 1917, Dublin—then still part of the United Kingdom—was divided into numbered postal districts. These continue in use in a modified form by An Post, the postal service of the Republic of Ireland. In 1923, Glasgow was divided in a similar way to London, with numbered districts preceded by a letter denoting the compass point (C, W, NW, N, E, S, SW, SE).

In January 1932 the Postmaster General approved the designation of some predominantly urban areas into numbered districts. In November 1934 the Post Office announced the introduction of numbered districts (short postal codes) in "every provincial town in the United Kingdom large enough to justify it". Pamphlets were issued to each householder and business in ten areas notifying them of the number of the district in which their premises lay. The pamphlets included a map of the districts, and copies were made available at local head post offices. The public were "particularly invited" to include the district number in the address at the head of letters.

A publicity campaign in the following year encouraged the use of the district numbers. The slogan for the campaign was "For speed and certainty always use a postal district number on your letters and notepaper". A poster was fixed to every pillar box in the affected areas bearing the number of the district and appealing for the public's co-operation. Every post office in the numbered district was also to display this information. Printers of Christmas cards and stationery were requested to always include district numbers in addresses, and election agents for candidates in the upcoming general election were asked to ensure they correctly addressed the 100 million items of mail they were expected to post. Businesses were issued with a free booklet containing maps and listings of the correct district number for every street in the ten areas.

The ten areas were:

• Birmingham
• Brighton/Hove
• Bristol
• Edinburgh
• Glasgow
• Leeds/Bradford
• Liverpool
• Manchester/Salford
• Newcastle upon Tyne
• Sheffield

For example, Toxteth was Liverpool 8. A single numbering sequence was shared by Manchester and Salford: letters would be addressed to Manchester 1 or Salford 7 (lowest digits, respectively). Some Birmingham codes were sub-divided with a letter, such as Great Barr, Birmingham 22 or Birmingham 22a, as can still be seen on many older street-name signs.

Modern postcode system

The Post Office experimented with electromechanical sorting machines in the late 1950s. These devices presented an envelope to an operator, who would press a button indicating which bin to sort the letter into. Postcodes were suggested to increase the efficiency of this process by removing the need for the sorter to remember the correct sorting for as many places. In January 1959 the Post Office analysed the results of a survey on public attitudes towards the use of postal codes, choosing a town in which to experiment with codes. The envisaged format was a six-character alphanumeric code with three letters designating the geographical area and three numbers to identify the individual address. On 28 July Ernest Marples, the Postmaster General, announced that Norwich had been selected, and that each of the 150,000 private and business addresses would receive a code by October. Norwich had been selected as it already had eight automatic mail sorting machines in use. The original Norwich format consisted of "NOR", followed by a space, then a two-digit number (which, unlike the current format, could include a leading zero), and finally a single letter (instead of the two final letters in the current format).

In October 1965, Tony Benn as Postmaster General announced that postal coding was to be extended to the rest of the country in the next few years.

On 1 May 1967 postcodes were introduced in Croydon. The many postcodes for central Croydon began with "CRO", while those of the surrounding post towns with CR2, CR3 and CR4. The uniform system of a set of three final characters after the space (such as 0AA, known as the inward code) was adopted. This was to be the beginning of a ten-year plan, costing an estimated £24 million. Within two years it was expected that full coding would be used in Aberdeen, Belfast, Brighton, Bristol, Bromley, Cardiff, Coventry, Manchester, Newcastle upon Tyne, Newport, Reading, Sheffield, Southampton and the Western district of London. By 1967, codes had been introduced to Aberdeen, Southampton, Brighton and Derby. In 1970, codes were introduced to the London Western and North Western postal districts. In December 1970, much Christmas mail was postmarked with the message "Remember to use the Postal Code" although codes were used to sort mail in only a handful of sorting offices.

During 1971, occupants of addresses began to receive notification of their postcode. Asked in the House of Commons about the completion of the coding exercise, the Minister of State for Posts and Telecommunications (whose role superseded that of Postmaster General in 1969), Sir John Eden, stated that it was expected to be completed during 1972. The scheme was finalised in 1974 when Norwich was completely re-coded but the scheme tested in Croydon was sufficiently close to the final design for it to be retained, with CRO standardised as CR0 (district zero) thus removing the need to create a CR1 district.

A quirk remained: the central Newport (Gwent) area was allocated NPT at a similar time to Croydon becoming CRO, and surrounding areas were (as today) allocated NP1–NP8. NPT lasted until the end of 1984 when it was recoded NP9.

Girobank's GIR 0AA was the last domestic postcode with a fully alphabetical outward code. That code no longer exists in the Royal Mail's PAF system, but was taken over by the bank's current owners, Santander UK.

Adaptation of earlier systems into national system

When the national postcode system was introduced, many existing postal districts were incorporated into it, so that postcodes in Toxteth (Liverpool 8) start with L8. The districts in both Manchester and Salford gained M postcodes, so Salford 7 became M7 and so on (and similarly in Brighton and Hove, both using the prefix BN). The old coding lives on in a small number of street signs with (for example) "Salford 7" at the bottom. In other cases, the district numbers were replaced with unrelated numbers. In Glasgow many of its G-prefixed numbers are not used following the transposition of the earlier compass point distrcits to "G" districts: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. In London (as postally defined), 1917-created postal districts are mapped unchanged today despite Greater London, created in April 1965, covering a much larger administrative area. The London post town covers 40% of Greater London and the remaining 60% of Greater London's area has postcodes referring to 13 other post towns. Additionally, there were too few postcodes to adequately cover districts in central London (particularly in the WC and EC areas), so these were subdivided with a letter suffix rather than being split into new numbered districts so as to retain the familiar codes.

GB postcodes available as OpenData

Prior to 1 April 2010, the Royal Mail licensed use of the postcode database for a charge of about £4,000 per year. Following a campaign and a government consultation in 2009, the Ordnance Survey released Code-Point Open, detailing each current postcode in Great Britain together with a geo-code for re-use free of charge under an attribution-only licence (Open Government Licence as part of OS OpenData).

Postcodes linked to a variety of UK geographies

The Office for National Statistics (ONS Geography) maintains and publishes a series of freely available, downloadable postcode products that link all current and terminated UK postcodes to a range of administrative, health, statistical and other geographies using the Code-Point Open grid reference.

Formatting

Overview

The postcodes are alphanumeric, and are (possibly uniquely) variable in length: ranging from six to eight characters (including a space). Each postcode is divided into two parts separated by a single space: the outward code and the inward code respectively. The outward code includes the postcode area and the postcode district, respectively. The inward code includes the postcode sector and the postcode unit respectively. Examples of postcodes are "SW1W 0NY", "PO16 7GZ", "GU16 7HF", and "L1 8JQ".

POSTCODE
Outward code		Inward code
Area	District	Sector	Unit
SW	1W	0	NY

Outward code

The outward code is the part of the postcode before the single space in the middle. It is between two and four characters long. Examples of outward codes are "L1", "W1A", "RH1", "RH10" or "SE1P". A few outward codes are non-geographic, not divulging where mail is to be sent. These postcodes cannot be used for navigation purposes.

Postcode area

The postcode area is part of the outward code. The postcode area is either one or two characters long and is alphabetical, with there being 121 such areas. Examples of postcode areas are "L" for Liverpool, "RH" for Redhill and "EH" for Edinburgh. A postal area may cover a wide area, for example "RH" covers various settlements in eastern Surrey and north eastern West Sussex, and "BT" (Belfast) covers the whole of Northern Ireland.

Postcode district

The postcode district is one digit, two digits or a digit followed by a letter.

Inward code

The inward code is the part of the postcode after the single space in the middle. It is three characters long. The inward code assists in the delivery of post within a postal district. Examples of inward codes are "0NY", "7GZ", "7HF", or "8JQ".

Postcode sector

The postcode sector is made up of a single digit (the first character of the inward code). Most postcode areas do not use all of the sectors 0-9 in order to allow for the possibility of more sectors being added in the face of new development. Rather, in the initial allocation of postcodes, neighbouring postcode districts were often assigned to contain the ten sectors between them. For example, across the three postcode districts BS6-8 (which are next to each other in north west Bristol), sectors 1-4 were assigned to BS8, sectors 5-7 were assigned to BS6, and sectors 8-9 and 0 were assigned to BS7 (more recent changes have resulted in all three of those areas now having a sector 9).

Postcode unit

The postcode unit is two characters added to the end of the postcode sector. A postcode unit generally represents a street, part of a street, a single address, a group of properties, a single property, a sub-section of the property, an individual organisation or (for instance Driver and Vehicle Licensing Agency) a subsection of the organisation. The level of discrimination is often based on the amount of mail received by the premises or business.

Validation

The format is as follows, where A signifies a letter and 9 a digit:

Format	Coverage	Example
AA9 9AA	All postcodes except those listed below	CR2 6XH
AA99 9AA		DN55 1PT
A9 9AA	B, E, G, L, M, N, S, W	M1 1AE
A99 9AA		B33 8TH
A9A 9AA	London postcode districts: E1, N1, W1	W1A 0AX
AA9A 9AA	London postcodes: WC postcode area; and districts EC1–EC4, NW1W, SE1P, SW1	EC1A 1BB

Notes:

• As all formats end with 9AA, the first part of a postcode can easily be extracted by ignoring the last three characters.
• Areas with only single-digit districts: BL, BR, FY, HA, HD, HG, HR, HS, HX, JE, LD, SM, SR, WC, WN, ZE (although WC is always subdivided by a further letter, e.g. WC1A)
• Areas with only double-digit districts: AB, LL, SO (for AB this arose from decoding of the original five districts AB1-AB5 by adding a second digit, to enable additional postcodes to become available, thus AB1 was divided into AB10-AB16).
• Areas with a district '0' (zero): BL, BS, CM, CR, FY, HA, PR, SL, SS (BS is the only area to have both a district 0 and a district 10)
• The following central London single-digit districts have been further divided by inserting a letter after the digit and before the space: EC1–EC4 (but not EC50), SW1, W1, WC1, WC2 and parts of E1 (E1W), N1 (N1C and N1P), NW1 (NW1W) and SE1 (SE1P).
• The letters Q, V and X are not used in the first position.
• The letters I, J and Z are not used in the second position.
• The only letters to appear in the third position are A, B, C, D, E, F, G, H, J, K, P, S, T, U and W when the structure starts with A9A.
• The only letters to appear in the fourth position are A, B, E, H, M, N, P, R, V, W, X and Y when the structure starts with AA9A.
• The final two letters do not use C, I, K, M, O or V, so as not to resemble digits or each other when hand-written.
• Postcode districts are one of ten digits: 0 to 9, with 0 only used once 9 has been used in a post town, save for Croydon (see above).
• Postcode sectors can also be one of ten digits: 0 to 9, though in some postcode areas the 0 is the beginning of the sequence (for example in LE), while in other areas it is the end of the sequence (ie. 10, as in CV).

A postcode can be validated against a table of all 1.7 million postcodes in Code-Point Open. The full delivery address including postcode can be validated against the Royal Mail Postcode Address File (PAF), which lists 29 million valid delivery addresses, constituting most (but not all) addresses in the UK. A regular expression for validating UK postcodes is specified in the British Standards document BS 7666.

Post towns

All or part of one or more postcode districts are grouped into post towns. Larger post towns may use more than one postcode district, for example Crawley uses RH10 and RH11. In a minority of cases, a single number can cover two or more post towns – for example, the WN8 district includes Wigan and Skelmersdale post towns; and the GL17 district contains five post towns.

Special cases

Crown dependencies

The Channel Islands and the Isle of Man established their own postal administrations separate from the UK in 1969. Despite this, they adopted the UK-format postcodes in 1993–94: Guernsey using GY, the Isle of Man using IM, and Jersey using JE.

The independent jurisdiction of Sark was assigned a unique postcode district GY10 in 2011 to differentiate it from Alderney. The CEO of Guernsey Post, Boley Smillie, said "this has been a long time coming" and "... Sark should have had its own identity back then [when postcodes were adopted in 1993]".

British Overseas Territories

Some of the British Overseas Territories have postcodes that broadly follow the format of the UK postcode system or, in Gibraltar's case, adopts the UK format:

Postcode	Location
ASCN 1ZZ	Ascension Island
BBND 1ZZ	British Indian Ocean Territory
BIQQ 1ZZ	British Antarctic Territory
FIQQ 1ZZ	Falkland Islands
GX11 1AA	Gibraltar
PCRN 1ZZ	Pitcairn Islands
SIQQ 1ZZ	South Georgia and the South Sandwich Islands
STHL 1ZZ	Saint Helena
TDCU 1ZZ	Tristan da Cunha
TKCA 1ZZ	Turks and Caicos Islands

These were introduced because mail was often sent to the wrong place, e.g., to St Helena instead of St Helens, Merseyside or St Helens, Isle of Wight. and to Edinburgh instead of Edinburgh, Tristan da Cunha, and many online companies would not accept addresses without a postcode. Mail from the UK continues to be treated as international, not inland, and sufficient postage must be used.

Bermuda has developed its own postcode system, with unique postcodes for street and PO Box addresses, as have the Cayman Islands, Montserrat and the British Virgin Islands. Montserrat recently introduced postal codes, and a system has been under consideration in Gibraltar with the code GX11 1AA being introduced as the generic postcode for the territory in the interim.

The separate postal code systems for those territories are shown below:

Postcode	Location
AI-2640	Anguilla
KYn-nnnn (List)	Cayman Islands
MSR-nnnn (List)	Montserrat
VG-nnnn (List)	British Virgin Islands
aa nn or aa aa List	Bermuda

The British Sovereign Base Areas of Akrotiri and Dhekelia in Cyprus use Cypriot postal codes for civilian use. The British military use BFPO addresses.

British Forces Post Office (BFPO)

The British Forces Post Office (BFPO) provides a postal service to HM Forces separate from that provided by Royal Mail in the United Kingdom, with BFPO addresses used for the delivery of mail in the UK and around the world. BFPO codes such as "BFPO 801" serve the same function as postal codes for civilian addresses, with the last line of the address consisting of "BFPO" followed a space and a number of 1 to 4 digits.

For consistency with the format of other UK addresses, in 2012 BFPO and Royal Mail jointly introduced an optional alternative postcode format for BFPO addresses, using the new non-geographic postcode area "BF" and the notional post town "BFPO". Each BFPO number is assigned to a postcode in the standard UK format, beginning "BF1". Inward codes are assigned: 0 – Germany, 1 – UK, 2 – Rest of Europe, 3 – Rest of World, 4 – Ships and Naval Parties, 5 – Rest of World, Operations and Exercises, 6 – Rest of World, Operations and Exercises. The database was released commercially in March 2012 as part of the Royal Mail Postal Address File (PAF). A postcode is not required if the traditional "BFPO nnnn" format is used.

Non-geographic codes

Most postcodes apply to a geographic area but some are used only for routing and cannot be used for navigation or estimating distances. They are commonly termed “non-geographic” postcodes in the sense that refer delivery points that are allocated to specific addresses (and cannot be sued for SatNav systems). They are used for various purposes - "bulk mail" or large volumes, centralised scanning of inward mail, competitions, parcel returns, direct marketing and PO boxes. They are also referred to as "business service indicator addresses". They cover -

• Post boxes: e.g. in the Brighton BN postcode area, BN50-BN52 are allocated to PO Boxes in Brighton, Rottingdean and Hove.
• Post boxes in London: London postcode districts ending in -P, such as SE1P, are usually allocated to PO Boxes (though other letters are also used).
• Postcode district: many geographic postcode areas also postcode districts that are "non-geographic" and are allocated to specific recipients which are government departments or commercial businesses (e.g. WV98 and WV99 for the Department of Work and Pensions correspondence.[1]). These are referred to as Large User (LU) in the Postcode Address File.
• Postcode area BX: BX is entirely non-geographic, with all its codes independent of the location of the recipient.
• Postcode area XX: the XX postcode area is non-geographic and is allocated for parcel returns of large volume online retailers (and for medical sample testing during the COVID pandemic).

There appear to be no binding rules publicly available for numbering of such districts, but many non-geographic districts are allocated a number higher than the currently existing postcode districts and often in the range 90-99 (several government departments use the aa98 district ina. umber of postcode areas). This is possible in most cases as virtually all postcode areas have not allocated districts in that number range (B90-B98, BT92-BT94 and M90 seem to be the only geographic districts in this range). However, there are many non-geographic districts numbered outside this range (e.g. American Express has the postcode district BN88; in Glasgow G58 is allocated to National Savings, as part of a mnemonic postcode G58 1SB, though it is located in G43 postcode district; and also in Glasgow, G70 is allocated to HMRC which is physically located in G67).

Some postcodes in the "non-geographic" range are in fact geographic, but specific to the institution or entity and not part of the surrounding numbering sequence (and thus quite different from the neighbouring properties) - for example, EH99 1SP can be used with GPS mapping to locate and navigate to the Scottish Parliament (which is directly opposite the Palace of Holyrood House, EH8 8DX, and across the road from 7/4 Canongate, EH8 8BX) in the same way that SW1A 0AA can be used to navigate to the Houses of Parliament.

The letters HQ for the last two letters may also mean it is most likely a non-graphical postcode or that Royal Mail holds the mail where a redirection, bulk mail delivery or open and scan to email service is available. Girobank's headquarters in Bootle used the non-geographic postcode GIR 0AA. There is also a special postcode for letters to Santa/Father Christmas, XM4 5HQ.

Many non-geographic postcodes do not appear on Royal Mail's own online postcode finder tool or their Click and Drop online postage printing tool, which can add to confusion when responding to organisations that use such addresses. Likewise, delivery services or couriers other than Royal Mail may not be able to deliver to such non-physical addresses. The UK government provides for couriers alternative geographic addresses to their BX addresses.[2]

Special postcodes

Postcodes are allocated by Royal Mail's Address Management Unit and cannot be purchased or specified by the recipient. However, Royal Mail sometimes assigns semi-mnemonic postcodes (sometimes based on the actual geographic postcode district) to high-profile organisations.

Prominent examples are:

Postcode	Organisation
B1 1HQ	HSBC UK headquarters at 1 Centenary Square, Birmingham
BS98 1TL	TV Licensing (now changed to DL98 1TL)
BX1 1LT	Lloyds Bank formerly known as Lloyds TSB Bank—non-geographic address
BX2 1LB	Bank of Scotland (part of Lloyds Banking Group)—non-geographic address
BX3 2BB	Barclays Bank—non-geographic address
BX4 7SB	TSB Bank
BX5 5AT	VAT Central Unit of HM Revenue and Customs (Roman numeral "VAT" = "5AT")—non-geographic address
CF10 1BH	Lloyds Banking Group (formerly Black Horse Finance)
CF99 1NA	Senedd (formerly National Assembly for Wales)
CO4 3SQ	University of Essex (Square 3)
CV4 8UW	University of Warwick
CV35 0DB	Aston Martin after their sports cars named "DB"
DA1 1RT	Dartford F.C. (nicknamed The Darts)
DE99 3GG	Egg Banking (decommissioned in February 2018, after the closure of the bank )
DE55 4SW	Slimming World
DH98 1BT	British Telecom
DH99 1NS	National Savings certificates administration
E14 5HQ	HSBC headquarters at 8 Canada Square, Canary Wharf
E14 5JP	JP Morgan (Bank Street)
E16 1XL	ExCeL London
E20 2AQ	Olympic Aquatics Centre
E20 2BB	Olympic Basketball Arena
E20 2ST	Olympic Stadium
E20 3BS	Olympic Broadcast Centre
E20 3EL	Olympic Velodrome
E20 3ET	Olympic Eton Manor Tennis Courts
E20 3HB	Olympic Handball Arena (now the Copper Box)
E20 3HY	Olympic Hockey Stadium
E98 1SN	The Sun newspaper
E98 1ST	The Sunday Times newspaper
E98 1TT	The Times newspaper
EC2N 2DB	Deutsche Bank
EC2Y 8HQ	Linklaters headquarters at One Silk Street
EC4Y 0HQ	Royal Mail Group Ltd headquarters
EC4Y 0JP	JP Morgan (Victoria Embankment)
EH12 1HQ	NatWest Group headquarters
EH99 1SP	Scottish Parliament (founded in 1999)
G58 1SB	National Savings Bank (the district number 58 also approximates the outline of the initials SB)
GIR 0AA	Girobank (now Santander Corporate Banking)
HA9 0WS	Wembley Stadium
HP5 1WA	Inland Waterways Association (decommissioned when the IWA moved office in April 2023)
IV21 2LR	Two Lochs Radio
L30 4GB	Girobank (alternative geographic postcode)
LS98 1FD	First Direct bank
M50 2BH	BBC Bridge House
M50 2QH	BBC Quay House
N1 9GU	The Guardian newspaper
N81 1ER	Electoral Reform Services
NE1 4ST	St James' Park Stadium, Newcastle United
NG80 1EH	Experian Embankment House
NG80 1LH	Experian Lambert House
NG80 1RH	Experian Riverleen House
NG80 1TH	Experian Talbot House
PH1 5RB	Royal Bank of Scotland Perth Chief Office
RM11 1QT	Queen's Theatre, Hornchurch
PH1 2SJ	St Johnstone Football Club
S2 4SU	Sheffield United Football Club
S6 1SW	Sheffield Wednesday Football Club
S14 7UP	The World Snooker Championships at the Crucible Theatre, Sheffield; 147 UP refers to a maximum lead (from a maximum break) in snooker
S70 1GW	The Glass Works - retail and leisure centre in Barnsley town centre
SA99	Driver and Vehicle Licensing Agency—All postcodes starting with SA99 are for the DVLA offices in the Morriston area of Swansea, the final part of the postcode relates to the specific office or department within the DVLA
SE1 0NE	One America Street, the London headquarters of architectural firm TP Bennett
SE1 8UJ	Union Jack Club
SM6 0HB	Homebase Limited
SN38 1NW	Nationwide Building Society
SR5 1SU	Stadium of Light, Sunderland AFC
SW1A 0AA	House of Commons (Palace of Westminster; see below for House of Lords)
SW1A 0PW	House of Lords (Palace of Westminster; see above for House of Commons)
SW1A 1AA	Buckingham Palace (the Monarch)
SW1A 2AA	10 Downing Street (the Prime Minister)
SW1A 2AB	11 Downing Street (Chancellor of the Exchequer)
SW1H 0TL	Transport for London (Windsor House, 50 Victoria Street)
SW1P 3EU	European Commission and European Parliament office (European Union)
SW1W 0DT	The Daily Telegraph newspaper
SW1V 1AP	Apollo Victoria Theatre
SW11 7US	Embassy of the United States, London
SW19 5AE	All England Lawn Tennis and Croquet Club (Venue of the Wimbledon Championships)
TW8 9GS	GlaxoSmithKline
W1A 1AA	BBC Broadcasting House (independently notable postcode)
W1D 4FA	The former address of The Football Association (decommissioned in February 2010 after they moved location)
W1N 4DJ	BBC Radio 1 (disc jockey)
W1T 1FB	Facebook

The postcode printed on Business Reply envelopes (which do not require a stamp) often ends with the letters BR.

Post codes ending 1AA are usually allocated to post offices (Crown, main and sub-) such as NR3 1AA for Magdalen Street post office in Norwich. Some are defunct following disposal of former Post Office buildings (e.g. EH1 1AA in Edinburgh).

Operation

Sorting

Postcodes are used to sort letters to their destination either manually, where sorters use labelled frames, or increasingly with letter-coding systems, where machines assist in sorting. A variation of automated sorting uses optical character recognition (OCR) to read printed postcodes, best suited to mail that uses a standard layout and addressing format.

A long string of "faced" letters (i.e. turned to allow the address to be read) is presented to a keyboard operator at a coding desk, who types the postcodes onto the envelopes in coloured phosphor dots. The associated machine uses the outward codes in these dots to direct bundles of letters into the correct bags for specific delivery offices. With a machine knowledge of the specific addresses handled by each postal walk at each office, the bundles can be further sorted using the dots of the inward sorting code so that each delivery round receives only its own letters. This feature depends upon whether it is cost effective to second-sort outward letters, and tends to be used only at main sorting offices where high volumes are handled.

When postcodes are incomplete or missing, the operator reads the post town name and inserts a code sufficient for outward sorting to the post town, where others can further direct it. The mail bags of letter bundles are sent by road, air or train, and eventually by road to the delivery office. At the delivery office the mail that is handled manually is inward sorted to the postal walk that will deliver it; it is then "set in", i.e. sorted into the walk order that allows the deliverer the most convenient progress in the round. The latter process is now being automated, as the roll-out of walk sequencing machines continues.

Integrated Mail Processors

Integrated Mail Processors (IMPs) read the postcode on the item and translate it into two phosphorus barcodes representing the inward and outward parts of the postcode, which the machines subsequently print and read to sort the mail to the correct outward postcode. Letters may also be sequentially sorted by a Compact Sequence Sorter (CSS) reading the outward postcode in the order that a walking postman/woman will deliver, door to door. On such items the top phosphorous barcode is the inward part of the code, the bottom is the outward.

IMPs can also read RM4SCC items, as used in Cleanmail, a different format to the above.

Mailsort and Walksort

A newer system of five-digit codes called Mailsort was designed for users who send "a minimum of 4,000 letter-sized items". It encodes the outward part of the postcode in a way that is useful for mail routing, so that a particular range of Mailsort codes goes on a particular plane or lorry. Mailsort users are supplied with a database to allow them to convert from postcodes to Mailsort codes and receive a discount if they deliver mail to the post office split up by Mailsort code. Users providing outgoing mail sorted by postcode receive no such incentive since postcode areas and districts are assigned using permanent mnemonics and do not therefore assist with grouping items together into operationally significant blocks. Walksort was discontinued in May 2012.

Listings and availability

Further information: List of postcode areas in the United Kingdom, List of postcode districts in the United Kingdom, and List of post towns in the United Kingdom

There are approximately 1.7 million postcodes in the United Kingdom, the Channel Islands and the Isle of Man.

Each postcode is divided by a space into two parts. As mentioned above, the first part starts with the postcode area and ends with the postcode district. The second part begins with a single digit, which indicates the postcode sector, and ends with the postcode unit.

Postcode areas are also divided into several post towns, which predate the introduction of postcodes, with the London post town uniquely covering more than one postcode area.

As of June 2016, there are 124 postcode areas, 2,987 postcode districts, 11,192 postcode sectors, and 1,500 post towns. As of January 2021, 55,540 full postcodes in England and Wales contain only one household. Addresses receiving large volumes of mail are each assigned separate "large user" postcodes. But most postcodes are shared by several neighbouring properties, typically covering about 15 addresses.

Life-cycle of postcodes

There are also significant numbers of discontinued (terminated) codes. Each month some 2,750 postcodes are created and 2,500 terminated.

Component	Part	Example	Live codes	Terminated codes	Other codes	Total
Postcode area	Out code	YO	124	0	3	127
Postcode district	Out code	YO31	2,984	103	4	3,087
Postcode sector	In code	YO31 1	11,197	1,071	4	12,272
Postcode unit	In code	YO31 1EB	1,767,416	876,312	4	2,643,732
Postcode Addresses			Approx. 29,965,962

Postcode Address File (PAF)

The Address Management Unit of Royal Mail maintains an official database of UK postal addresses and postcodes in its Postcode Address File (PAF), which is made available under licence for a fee regulated by Ofcom. The PAF is commercially licensable and is often incorporated in address management software packages. The capabilities of such packages allow most addresses to be constructed solely from the postcode and house number. By including the map references of postcodes in the address database, the postcode can be used to pinpoint a postcode area on a map. PAF is updated daily.

On its website, Royal Mail publishes summary information about major changes to postcode sectors and postal localities (including post towns). Individual postcodes or postal addresses can be found using Royal Mail's Postcode and Address Finder website, but this is limited to 50 free searches per user per day.

Code-Point Open

A complete list of all current Great Britain postcodes, known as Code-Point Open, has been made available online (since 1 April 2010) by Ordnance Survey. Under the government's OS OpenData initiative, it is available for re-use without charge under an attribution-only licence. The Code-Point Open list includes median coordinates for each postcode but excludes postcodes in Northern Ireland and the Crown dependencies. Unlike the PAF products provided by Royal Mail, the Code-Point Open list does not include postal address text.

ONS Postcode Directory and National Statistics Postcode Lookup

The Office for National Statistics also produces postcode directories, under similar licence terms to the OS product. Both the ONSPD and NSPL contain Northern Ireland postcodes, with centroid coordinates in the OSI grid as opposed to the OSGB grid, although Northern Ireland postcodes are subject to a more restrictive licence permitting internal business use only. Postcodes for the Crown Dependencies are also included, without co-ordinates. A further difference is that non-current postcodes and dates of introduction and withdrawal of postcodes are included.

Other uses

See also: Postcode lottery

While postcodes were introduced to expedite the delivery of mail, they are useful tools for other purposes, particularly because codes are very fine-grained and identify just a few addresses. Among these uses are:

• With satellite navigation systems, to navigate to an address by street number and postcode
• By life insurance companies and pension funds to assess longevity for pricing and reserving
• By other types of insurance companies to assess premiums for motoring/business/domestic policies
• To determine catchment areas for school places or doctors' surgeries
• Finding the nearest branch of an organisation to a given address. A computer program uses the postcodes of the target address and the branches to list the closest branches in order of distance as the crow flies (or, if used in conjunction with street-map software, by road distance). This can be used by companies to inform potential customers where to go, by job centres to find jobs for job-seekers, to alert people of town planning applications in their area, and a great many other applications.

The phrase "postcode lottery" refers to the variation in the availability of services by region, though not always because of postcodes.

For these and related reasons, postcodes in some areas have become indicators of social status. Some residents have campaigned to change their postcode to associate themselves with a more desirable area, to disassociate with a poorer area, to reduce insurance premiums or to be associated with an area with a lower cost of living. In all these cases Royal Mail has said that there is "virtually no hope" of changing the postcode, referring to their policy of changing postcodes only to match changes in their operations.

Postcode areas rarely align with local government boundaries. The phenomenon whereby postcodes overlap administrative boundaries is known as 'straddling'. Some postcodes straddle England's borders with Wales and Scotland, such as CH and TD. This has led to British Sky Broadcasting subscribers receiving the wrong BBC and ITV regions, and newly licensed radio amateurs being given incorrect call signs.

Design Limitations

When postcodes were designed, there was essentially a single objective premised on improved mail sorting and delivery. As users of the postal system have become more diversified, with differing needs, and more delivery options have evolved, and the postcode system is used in different ways, the original postcode design concept has been adapted, but with resulting inconsistencies (that detract to some degree from the usefulness of the system, and the memorability of postcodes). Some of these are -

• variable length postcodes: postcodes vary in length from 5 to 7 characters (which cannot be validated without using complex software or having expert knowledge)
• "identifiers" for special purpose postcodes: there are no clear identifiers for special purpose postcodes (which cannot be used for navigation) to differentiate them from "geographic" postcodes
• exhaustion of postcodes: there is no apparent policy of allocating postcode districts so as to have a reserve for future demand.

None of these are identified as necessitating changes to the current system - the costs would certainly outweigh the benefits - but it seems that opportunities were missed to anticipate expansion and diversification of the system. Future-proofing may have involved -

Postcode length: issuing all postcodes with 7 characters. This would have required simple modification - first, the single letter postcode areas to have been two characters (possibly BM for B Birmingham, GW or GG for G Glasgow, LP or LV for L Liverpool, MN or MR or M Manchester, SH for S Sheffield, and in London EL for London East, NL for London North and WL for London West) and, second, all postcode districts to be 2 digits, numbered from 01 (such as NE01 1AA), or numbered only from 10 (such as NE10 1AA), up to 89, with London retaining the use of both two digits and digit/letter for subdivisions of postcode districts.

Even if London had a single postcode area, such as "LO", the number of districts exceeds 99 so the use of digit/letter combinations to name postcode districts would be necessary.

For all postcode areas, postcode district "aa00" could be reserved for post office use, or other special use, in each area.

With a standard four character postcode districts, some care would be needed in allocating districts (it may be preferable to avoid the postcode districts SN07, SW07 and SH17 for example, in the same way that some car registration numbers in the UK are withheld from issue, and some letter combinations are not issued by some European postal administrations because of historical connotations).

Special purpose postcodes: allocating all special purpose postcodes within a dedicated range would have "flagged" their status and also avoided confusion with "geographic" postcodes. The obvious range would have been XA to XY ("Z" is not used as the second character of the postcode area), as no geographic postcode area starts with "X". The use of mnemonics within this range would be possible, such as XB for "Box", XF for Forces/BFPO, XR for Returns, XM for Christmas mail, etc.).

Within geographic postcode areas, if needed, the range "AA90" to "AA99" could have been dedicated exclusively for large volume users, post boxes etc. thus using "9" as the flag. Alternatively, using postcode districts "AAnX" could be a clearer flag (and consistent with the current use of "XX" postcode area for (some) parcel returns. Postcodes in the format "AAnn nBX" could have been reserved for local postboxes (allowing up to 10 locations for postboxes within each postcode district).

Special purpose postcodes enable use of new delivery mechanisms such as lockers: these are allocated dedicated (and differentiated) postcodes to easily sort mail to locker destinations (e.g. Post Packup system in Luxembourg which overlays special postcodes for lockers to the existing postcode zones).

Postcode exhaustion: allocating postcodes based on there being a maximum initial allocation of (say) 75% of the available postcodes may have avoided the need for the creation of new postcode districts and/or recoding following exhaustion of available postcodes.

See also

• List of postcode areas in the United Kingdom
• List of postal codes (around the world)
• ACORN
• Address Point
• Postcode lottery
• RM4SCC—a machine-readable barcode version of the postcode and delivery point suffix
• Postcode Address File
• Unique Property Reference Number