UK postal codes are known as postcodes.

UK postcodes are alphanumeric. These codes were introduced by the Royal Mail over a 15-year period from 1959 to 1974 — the full list is now available electronically from the Royal Mail as the Postcode Address File. They have been widely adopted not just for their original purpose of automating the sorting of mail but for many other purposes such as insurance premium calculations and as a way to describe United Kingdom locations to route planning software.

UK postcodes are copyrighted, there is a project to help create a free postcode system - see [1]

However, as the format of the codes does not achieve its objective of primarily identifying the main sorting office and sub-office they have been supplemented by a newer system of five-digit codes called Mailsort — but only for mailings of 'a minimum of 4,000 letter-sized items'.^[1] Mail users who can deliver mail to the post office sorted by Mailsort code receive discounts but [bulk] delivery by postcode provides no such incentive.

Postcode history

The major cities of the UK have much older postcodes, now incorporated into the current system, than other areas. The first system of ten London postal districts identified by letters (W, WC, EC etc) was devised by Sir Rowland Hill and introduced in 1857 and 1858.^[2] The numbered subdivisions (W1, W2 etc) were a war-time measure and date from 1917. The 1917 subdivisions remain important, because they form the first part of the two-part modern postcode (so N1 1AA is an address in the old N1 district), and because they continue to be used by Londoners to refer to their districts.

The Post Office experimented with electromechnical sorting machines in the late 1950s. These devices would present 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.^[3]

In January 1959 the Post Office analysed the results of a survey on public attitudes towards the use of postal codes. The next step would be choose a town in which to experiment with coded addresses. The envisaged format was to be a six character alphanumeric code with three letters designating the geographical area and three numbers to identify the individual address.^[4] 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.^[5] The codes were in the form NOR followed by three digits.

In October 1965 it was confirmed that postal coding was to be extended to the rest of the country in the "next few years".^[6] On 1st May 1967 post codes were introduced in Croydon. The codes for central Croydon started with the letters CRO, and those of the surrounding post towns with CR2, CR3 and CR4. This was to be the beginning of a ten year plan, costing an estimated £24 million. Within two years it was expected that 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.^[7] By 1967 codes had been introduced to Aberdeen, Southampton, Brighton and Derby.^[8] In 1970 codes were introduced to the Western and North West London areas.^[9] In December 1970 Christmas mail was franked with the message "Remember to use the Postcode", although codes were only used to sort mail in a handful of sorting offices.^[10]

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 Postmaster General, Sir John Eden stated it was expected to be completed during 1972.^[11]

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.^[12] Newport was originally allocated NPT, in a similar way to Norwich and Croydon, with the surrounding towns allocated NP1-NP8. This lasted into the mid 1980s when for operational reasons (NPT being non-standard, and too similar to NP7) it was recoded.

The legacy of the Croydon trial can still be seen today:

• CR0 was the only postal district with a zero in that position: all others start with 1. This caused one of the PAF (see above) software products produced by the Royal Mail themselves to misbehave slightly. Subsequently, the "zeroth" district has been used in some other postcode areas, such as Bolton, Harrow, Slough and the Dengie peninsula in Essex.
• A separate postal "district", CR9 is used for large users and PO Box holders. This policy has been used elsewhere, with normal postcodes "growing" upwards from district 1 and large-user postcodes "growing" downwards from district 99.
• The CR0 district contains far more addresses than any other postal district in the country.
• CR1 has never been used — possibly left spare for rationalisation. (The other CR districts, CR2 etc. were coded later and conform to the general standards.)
• There was at one point a movement to change all CR0 postcodes to CR1, but this was rejected.
• CR0 is often incorrectly written as CRO, although in some type faces the digit '0' and letter 'O' are identical -- the problem is exacerbated as it is often pronounced 'Sea Arr Oh' rather than 'Sea Arr Zero'.

Format

The format of UK postcodes is generally:

A9 9AA

A99 9AA

A9A 9AA

AA9 9AA

AA99 9AA

AA9A 9AA

where A signifies a letter and 9 a digit. It is a hierarchical system, working from left to right — the first letter or pair of letters represents the area, the following digit or digits represent the district within that area, and so on. Each postcode generally represents a street, part of a street, or a single premises. This feature makes the postcode useful to route planning software.

The part of the code before the space is the outward code or out code used to direct mail from one sorting office to the destination sorting office, while the part after the space is the inward code or in code used to sort the mail into individual delivery rounds, each separate code usually identifying the address to within 80 properties (with an average of 14 properties per postal code), although a large business may have a unique code. The outward code can be split further into the area part (letters identifying one of 124 postal areas) and the district part (usually numbers); similarly, the inward code is split into the sector part (number) and the unit part (letters).

Name	Location	Component Format	Number
postcode area	out code	A or AA	124
postcode district	out code	9, 99 or 9A	3,064
sector	in code	9	11,598
unit	in code	AA	1,780,000
Postcode Addresses			27,000,000

The letters in the outward code may give some clue to its geographical location (but see London below). For example, BS indicates Bristol, G indicates Glasgow and CF indicates Cardiff; see List of postcode areas in the United Kingdom for a full list. Although BT indicates Belfast, it covers the whole of Northern Ireland. The letters in the inward code, however, are restricted to the set ABDEFGHJLNPQRSTUWXYZ (excluding CIKMOV), which generally do not resemble digits or each other when hand-written.

There are at least two exceptions (other than the overseas territories) to this format:

• the postcode for the formerly Post Office-owned Girobank is GIR 0AA.
• the postcode for correctly addressed letters to Father Christmas is SAN TA1^[13]

Greater London postcodes

Main article: London postal district

In the London Postal Area postcodes are slightly different, being based on the 1856 system of Postal Districts which was refined in 1917 by numbering the 163 Sub-Districts; predating by many years the introduction of postcodes in the 1960s:

• In central London, WC and EC (West Central and East Central)
• In the rest of the London Postal Area, N, NW, SW, SE, W and E.

The London postal districts rarely coincide with the boundaries of the London boroughs (even the former, smaller Metropolitan Boroughs). The numbering system appears arbitrary on the map: for example, NW1 is close to central London, but NW2 is a long way out. This is because (after starting with 1 for the area containing the main sorting office) they were numbered alphabetically by the name of the main sorting office.

The area covered by the London postal districts was somewhat larger than the County of London, and included parts of Kent, Essex, Surrey, Middlesex and Hertfordshire. In 1965 the creation of Greater London caused this situation to be reversed as the boundaries of Greater London went beyond most of the existing London postal districts.

Those places not covered by the existing districts received postcodes as part of the national coding plan, so the postcode areas of "EN" Enfield, "KT" Kingston upon Thames, "HA" Harrow, "UB" Uxbridge", "TW" Twickenham, "SM" Sutton, "CR" Croydon, "DA" Dartford, "BR" Bromley, "RM" Romford and "IG" Ilford cross administrative boundaries and cover parts of neighbouring counties as well as parts of Greater London.

A further complication is that in some of the most central London areas, a further gradation has been necessary to produce enough postcodes, giving codes like EC1A 1AA.

While most postcodes are allocated by administrative convenience, a few are deliberately chosen. For example in Westminster:

• SW1A 0AA - House of Commons
• SW1A 0PW - House of Lords, Palace of Westminster
• SW1A 1AA - Buckingham Palace
• SW1A 2AA - 10 Downing Street, Prime Minister and First Lord of the Treasury
• SW1A 2AB - 11 Downing Street, Chancellor of the Exchequer
• SW1A 2HQ - HM Treasury headquarters

Other areas' postcodes

Until the 1960s, Postal Areas such as Belfast, Birmingham, Bradford, Edinburgh, Glasgow, Leeds, Liverpool, Manchester, Salford, Newcastle upon Tyne and Sheffield were divided into numbered Postal Districts, e.g. Toxteth in Liverpool was Liverpool 8. When the national postcode system was introduced, these were incorporated into it, so that postcodes in Toxteth start with L8. A similar system is still used in the Republic of Ireland for Dublin's postal districts.

Some Birmingham codes were sub-divided, with a letter, such as Great Barr, Birmingham 22 or Birmingham 22a^[14] - as can still be seen on many older street-name signs.

A single numbering sequence was split between Manchester and Salford. Letters would be addressed to Manchester 1 or Salford 4. However in the 1960s, all the districts in both Manchester and Salford gained "M" postcodes, so "Salford 4" became M4, etc., much to the chagrin of Salfordians. The old coding lives on in a handful of street signs which are still embossed with "Salford 4" etc, at the bottom.

Glasgow shared with London a distinction from all other UK cities as it had compass postal districts due to its claimed status as the Second City of the British Empire, i.e., C, W, NW, N, E, S, SW, SE. When postcodes were introduced these were mapped into the new 'G' postcode area thusly: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on.

Validation

The consequence of the complexity outlined above is that for almost every rule concerning UK postcodes, an exception can be found. Automatic validation of postcodes on the basis of pattern feasibility is therefore almost impossible to design, and the system contains no self-validating feature such as a check digit. Completely accurate validation is only possible by attempting to deliver mail to the address, and verifying with the recipient.

Validation is usually performed against a copy of the "Postcode Address File" (PAF), which is generated by the Royal Mail and contains about 27 million UK commercial and residential addresses. However, even the PAF cannot be relied on as it contains errors, and because new postcodes are occasionally created and used before copies of the PAF can be distributed to users.

It is possible to validate the format of a postcode using the rules described in British Standard BS 7666.^[15] In general, the format is one of "A9 9AA", "A99 9AA", "AA9 9AA", "AA99 9AA", "A9A 9AA" or "AA9A 9AA", where A is an alphabetic character and 9 is a numeric character. There are restrictions on the set of alphabetic characters dependent on the position they are in.

As can be seen, the first character is always alphabetical and the final three characters are always a numeric character followed by two alphabetic characters.

A regular expression to implement the BS 7666 rules in a basic fashion is provided in the BS7666 schema:^[16]

[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][A-Z-[CIKMOV]]{2}

A more complex regular expression is also given in the comments of the schema, which implements full checking of all the stated BS 7666 postcode format rules. That regular expression can be restated as a "traditional" regular expression:

(GIR 0AA|[A-PR-UWYZ]([0-9]{1,2}|([A-HK-Y][0-9]|[A-HK-Y][0-9]([0-9]|[ABEHMNPRV-Y]))|[0-9][A-HJKS-UW]) [0-9][ABD-HJLNP-UW-Z]{2})

The BS 7666 rules do not match British Forces Post Office postcodes, which have the format "BFPO NNN" or "BFPO c/o NNN", where NNN is 1 to 4 numerical digits.

Non-geographic postcodes

Almost all postcodes map directly to a geographic area, but there are some which are used simply for routing, mostly for the purposes of direct marketing, and cannot be used for navigation or distance-finding applications.^[17]

These codes include BS98, BS99, BT58, NE98, NE99 and WC99.

There is an additional entirely non-geographic outward code BX, from which postcodes can be allocated entirely independently of the geographic location of the recipient (and which can be retained in the event of the customer moving)

Within Royal Mail, outward codes beginning XY are used internally as routing codes to route mis-addressed mail, and to route international outbound mail.

Application

The PAF is commercially licenseable and is often incorporated in address management software packages. The capabilities of such packages allow an address to be constructed solely from the postcode and house number for most addresses. By including the map references of postcodes in the address database, the postcode can be used automatically to pinpoint a postcode area on a map. See [2] for an example of this in practice.

Northern Ireland

Northern Ireland was the last part of the UK to be postcoded with all postcodes beginning BT, between 1970 and 1974. While Belfast was already divided into postal districts, rural areas known as townlands posed an additional problem, as (at the time) many roads were not named, and houses were not numbered. Consequently, many people living in such areas shared the same postal address, which still occurs in the Republic of Ireland. Today the majority of roads in Northern Ireland are named (notable exceptions are in Fermanagh) and most houses (even in rural areas) are allocated a number. Those that aren't can be uniquely identified by a house name.

Crown dependencies

The Channel Islands (Jersey and Guernsey) and the Isle of Man established their own postal administrations separate from the UK in 1969, and did not adopt postcodes until the early 1990s. Their postcodes follow the UK format, with Jersey being postcode area JE, Guernsey GY, and Isle of Man IM.

In the Isle of Man some choose to use EV as opposed to IM. EV stands for Ellan Vannin, the name of the Isle of Man in the Manx language, and is sometimes used when an address is written in Manx.

British Forces Post Office (BFPO)

The British Forces Post Office (BFPO) is an agency that provides a postal service to HM Forces, separate from that provided by Royal Mail in the United Kingdom. BFPO addresses are 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.

Overseas territories

Some of the UK's overseas territories have their own postcodes, which are used for all addresses in those territories:

• Ascension Island: ASCN 1ZZ
• British Antarctic Territory: BIQQ 1ZZ
• British Indian Ocean Territory: BBND 1ZZ
• Falkland Islands: FIQQ 1ZZ
• Pitcairn Islands: PCRN 1ZZ
• Saint Helena: STHL 1ZZ
• South Georgia and the South Sandwich Islands: SIQQ 1ZZ
• Tristan da Cunha: TDCU 1ZZ^[18]
• Turks and Caicos Islands: TKCA 1ZZ

These were introduced because mail was often sent to the wrong place, e.g: St Helena to St Helens, Merseyside, and Ascension Island to Asunción, Paraguay. In addition, 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, the UK's most populous remaining overseas territory, has developed its own, entirely separate, postcode system, with unique postcodes for street and PO Box addresses, as have the Cayman Islands.

Anguilla, British Virgin Islands, Montserrat do not have postcodes, although a postcode system is under consideration in Gibraltar.^[19]

See also

• List of postal districts in the United Kingdom
• List of postcode areas in the United Kingdom
• List of postal codes for worldwide postal code definitions.
• ACORN (geodemography)
• ADDRESS-POINT
• Australia Post
• British Forces Post Office
• P² People & Places (geodemography)
• Post town
• Postcode Address File
• Postcode lottery
• RM4SCC a machine readable barcode version of the postcode
• Royal Mail
• UK topics
• ZIP Codes - The US version

References

External links

• CSV file mapping postcodes to long/lat
• More detailed explanation of the postcode system
• [https://www.royalmail.com/portal/rm/postcodefinder?pageId=pol_login&catId=400145&cs=1 Postcode finder in the United Kingdom]
• Postcode definition from the UK Government Data Standards Catalogue
• British postal codes - this PDF document from the Universal Postal Union explains the system and shows all allowable UK postcode formats
• How Tristan da Cunha got its postcode
• Article on postcode affecting insurance premiums
• Changes to UK postcodes
• Free data files listing all UK postal regions and neighbouring regions, for geographical result ordering
• www.freethepostcode.org - A community effort to build a public domain database of UK postcodes against GPS coordinates.
• UK Post Code distance calculation information - Geographical Post Code information and web site integration techniques
• www.npemap.org.uk - A community effort to build a public domain database of UK postcodes via clicks on maps from the 1950s (for those who don't have a GPS device).
• A free user contributed UK Postcode database
• www.pagemost.com A free website where you can search for a postcode and see a map of the surrounding area including place names and attractions. You can link directly to a page by appending a postcode to the url for example www.pagemost.com/SW1A1AA
• Map of British postcode areas