Postcodes in the United Kingdom

The postal codes used in the United Kingdom are known as postcodes.^[1] They are alphanumeric and were introduced by the Royal Mail over a 15-year period from October 1959 to 1974.^[2] A full postcode is known as a "postcode unit" and usually corresponds to a limited number of addresses or a single large delivery point.^[1] Postcode units consist of between five and seven characters, separated into two parts by a space; there are approximately 1.8 million postcode units.^[1] The first part of the postcode unit is known as a "postcode district",^[1] or outward code, and usually corresponds to all or part of a post town. Postcode districts with the same one or two character prefixes are grouped into 124 postcode areas.^[1] Postcodes have been adopted for a wide range of purposes in addition to aiding the automating sorting of the mail; and are used to calculate insurance premiums, designate destinations in route planning software and are used as the lowest level of aggregation in census enumeration. Postcode data are stored, maintained and periodically updated in the Postcode Address File database, along with the full address data for around 27.5 million delivery points.^[1] An earlier system of postal districts was implemented in London and other large cities from 1857. In London this system was refined in 1917 to include numbered subdivisions, extending to the other cities in 1934. These earlier districts were later incorporated into the national postcode system.

Overview

The postcodes are alphanumeric and between five and eight characters long (including a single space separating the outward and inward parts of the code), e.g. the code for the House of Commons is SW1A 0AA. These codes were introduced by the Royal Mail between 1959^[3] and 1974.^[2][4] They have been widely adopted not just for their original purpose of automating the sorting of mail, but for many other purposes — see postcode lottery.

The 'Outward' part of the postcode denotes the postal district - for example RH for the Redhill area, and then the following number distinguishes the post town - broadly speaking the Delivery Office which services the local area. So RH1 is Redhill itself, RH10 is Crawley. With larger towns there may be more than one number in the outward section - Crawley includes RH10 and RH11. The reverse situation is uncommon but can also occur, with a single postal district lying within more than one post town. The 'Inward' part denotes particular parts of the town / Delivery Office area, with the first part - the number - being a sector, and the final two letters denoting a property or group of properties within that area. In the case of a large office block, for example, the 'Inward' part of the code may denote just a part of the office block, or often just a single company within that block (particularly where the company receives a large amount of mail). In some cases (for instance DVLA) the "inward" code may serve to direct mail to different parts of the same organisation.^[5]

A series of five-digit codes may also be used on business mail. This is called Mailsort – but is only available for mailings of 'a minimum of 4,000 letter-sized items'.^[6] Discounts are available for such bulk mailings based on the type of mail and how pre-sorted it is.

Development

The Post Office experimented with electromechanical sorting machines in the late 1950s.^[7] 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.^[8] 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 to choose a town in which to experiment with coded addresses. The envisaged format was a six character alphanumeric code with three letters designating the geographical area and three numbers to identify the individual address.^[9] 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.^[10] The codes were prefixed NOR.

In October 1965 it was confirmed that postal coding was to be extended to the rest of the country in the "next few years".^[11] On 1 May 1967 postcodes were introduced in Croydon. The codes for central Croydon started with the three 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.^[12] By 1967 codes had been introduced to Aberdeen, Southampton, Brighton and Derby.^[13] In 1970 codes were introduced to the Western and North Western London areas.^[14] 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.^[15]

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 that it was expected to be completed during 1972.^[16] 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).^[17] The central Newport area was originally allocated NPT, in a similar way to Norwich and Croydon, with the surrounding areas allocated NP1–NP8. This lasted until the end of 1984 when for operational reasons (NPT being non-standard and too similar to NP7) it was recoded to NP9.^[18] Girobank's GIR 0AA survives as the only domestic postcode with a wholly alphabetical outward code.

Earlier postal districts

London

The London post town covers 40% of Greater London. When introduced in 1857/8 it was divided into ten areas with codes indicating the areas they covered: EC, WC, N, NE, E, SE, S, SW, W, and NW. The S and NE sectors were later abolished and in 1917, as a wartime measure to improve efficiency, the districts were further subdivided with a number applied to each sub-district. This was achieved by designating the area served directly by the head office in each district as "1" and then allocating the other numbers according to the alphabetical position of the names of the locations of each delivery office (e.g. N2 East Finchley, N3 Finchley, N4 Finsbury Park etc.).

Other large towns

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

Following the successful introduction of postal districts in London, the system was gradually extended to a number of other large towns in the United Kingdom. Liverpool was divided into Eastern, Northern, Southern and Western districts in 1864/65, and Manchester and Salford into eight numbered districts in 1867/68.^[17]

In 1917 Dublin was divided into numbered postal districts. These continue in use in a modified form by the postal service of the independent 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).^[17]

In January 1932 the Postmaster General approved the division of a number of large towns into numbered districts.^[17] In November 1934 the Post Office announced the introduction of the districts in "every provincial town in the United Kingdom large enough to justify it". Pamphlets were issued to each householder and business in the ten selected areas notifying them of the number of the district in which their premises lay. The pamphlets also included a map of the divisions, 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 all private or business letters.^[19] A publicity campaign in the following year was made to encourage 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. In addition, businesses were issued with a free booklet containing maps and listings of the correct district number for every street in the ten areas.^[20]

The ten areas were:^[20]

Each was divided into numbered postal districts, e.g. Toxteth in Liverpool was Liverpool 8. A single numbering sequence was split between Manchester and Salford. Letters would be addressed to Manchester 1 or Salford 4. Some Birmingham codes were sub-divided, with a letter, such as Great Barr, Birmingham 22 or Birmingham 22a^[21] - as can still be seen on many older street-name signs.

Adaptation into national system

When the national postcode system was introduced, the 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 4" became M4, etc. The old coding lives on in a small number of street signs which are still embossed with "Salford 4" etc., at the bottom. In Glasgow postcodes were mapped into the new 'G' postcode area: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. In London the 1917 postal districts mapped directly to the new postcode districts. The remaining 60% of Greater London was allocated postcodes under the national plan.

Some older road signs in London's Hackney area still indicate a North East(NE) postcode today!

Operation and application

Format

Postcode areas of the United Kingdom

The format of UK postcodes (with the exception of GIR 0AA) is as follows, where A signifies a letter and 9 a digit:

It is a hierarchical system, working from left to right:

• The two to four characters before the space comprise the outward code or out code intended to direct mail from the sorting office to the delivery office:
  • The first letter or pair of letters represents the postcode area.
  • The following number, from 0 to 99, determines the postcode district within that area.
    • Areas which have only single-digit districts: BR, FY, GY, HA, HD, HG, HR, HS, HX, JE, LD, SM, SR, WC, WN, ZE.
    • Areas which have only double-digit districts: AB, LL, SO.
    • Only a few areas have a district 0 (zero): BL, CM, CR, FY, HA, PR, SL, SS; none of these areas also has a district 10, and district 0 is sometimes treated as though it were the tenth district in the area and sorted after district 9.
    • In central London, some overcrowded single-digit postcode districts have been further divided by inserting a letter after the digit and before the space. This applies to all of EC1–EC4 (but not EC50), SW1, W1, WC1 and WC2; and to part of E1 (E1W), N1 (N1C and N1P), NW1 (NW1W) and SE1 (SE1P). All letters in the set ABCDEFGHJKMNPRSTUVWXY are currently used as the trailing letter in one or more divided districts, excluding the five letters ILOQZ.
    • The term "postcode district" is ambiguous in common usage, as it may refer either collectively to all the alphabetical and non-alphabetical parts in a (former) district, or only to one such part. For example, a reference to N1 might be intended either to include or to exclude N1C and N1P, depending on context, and N1C might be said to be a district or (loosely) part of the N1 district.
• The outward code is followed by a space.
• The three characters after the space comprise the inward code or in code intended to sort mail at the final delivery office:
  • The first character after the space is a digit from 0 to 9 which determines the postcode sector. Originally, Royal Mail sorted sector 0 after 9 instead of before 1, effectively treating it as the 10th not the 1st sector label.
  • The final two letters form the postcode unit. The letters in the inward code are restricted to the set ABDEFGHJLNPQRSTUWXYZ, excluding the six letters CIKMOV so as not to resemble digits or each other when hand-written.

Each postcode unit generally represents a street, part of a street, or a single address. This feature makes postcodes useful to route planning software.

The letters in the outward code give some clue to its approximate geographical location. For example, L indicates Liverpool, EH indicates Edinburgh and AB indicates Aberdeen; see List of postcode areas in the United Kingdom for a full list. Most postcode areas outside London cover many towns and localities beyond the city after which they are named. For instance, although BT indicates Belfast, it covers the whole of Northern Ireland.

Non-geographic codes

Most postcodes map directly to a geographic area but some are used only for routing and cannot be used for navigation or distance-finding applications.^[26] Non-geographic postcodes are often used for direct marketing and PO boxes. Some postcode sectors or districts are set aside solely for non-geographic postcodes, including BS98, BS99, BT58, E98, NE98, NE99 and WC99.

Girobank's headquarters in Bootle uses the non-geographic postcode GIR 0AA which is unique in format. There is also a non-geographic postcode area, BX, solely for non-geographic addresses. Postcodes beginning with BX follow the standard format but are allocated independently of the location of the recipient and can be retained in the event of the recipient moving. Prominent users include Lloyds TSB^[24] and HM Revenue and Customs.^[27] There is a special postcode for letters to Father Christmas: SAN TA1.^[28]

Special postcodes

Postcodes are usually allocated solely for logistical convenience, however there are a few informal exceptions.

Britain's constitutional hierarchy is unofficially reflected in the ordering of the following three postcodes:

Organisations which receive enough post to justify having a dedicated postcode also, in a small proportion of cases, have their organisation name reflected in the last part of the code. Prominent examples include:

Automation

The Integrated Mail Processors - IMP's - that Royal Mail use, read the postcode on the item, and translates it into two phosphorous barcodes, unique to 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 sequently sorted by a CSS machine, reading the outward postcode, in the order that a walking postman will deliver, door to door. The top phosphorous barcode is the inward part of the code, whilst the bottom is the outward, on such items.

IMP's can also read RM4SCC items, as used in Cleanmail, a completely different format to the above.

Use by third parties

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. The PAF is constantly updated with around 4,000 postcodes added each month and 2,000 existing postcodes terminated.^[39]

Crown Dependencies

The Channel Islands and the Isle of Man established their own postal administrations separate from the UK in 1969. Despite this, when they were subsequently postcoded they adopted the UK format, with Guernsey postcoded in 1993 using area GY, the Isle of Man postcoded the same year using area IM, and Jersey postcoded in 1994 using area JE.^[40]

British Forces

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.

It is based in North West London.

Overseas Territories

Some of the UK's overseas territories have their own postcodes, each postcode covering all addresses in the relevant territory:

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. Royal Mail's Heathrow centre collects all live underpaid mail for surcharging, and there is a reciprocal arrangement with Postal Services around the world to collect. An agreed payment based on volumes is made, year on year. Other forms of postage are collected at local Mail Centres, but Heathrow collects those that still get forwarded to them, that manage to arrive there. 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^[44] and the British Virgin Islands.^[45] Montserrat and Gibraltar do not have postcodes, although a postcode system has been under consideration in Gibraltar.^[46] Postcodes are not used in the Turks and Caicos Islands and the TKCA 1ZZ designation is generally unknown.

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, covered by more than 1.7 million postcodes.^[47] 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.^[48] 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 is 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-HJKPS-UW]) [0-9][ABD-HJLNP-UW-Z]{2})

British Forces Post Office postcodes do not follow the BS 7666 rules, but have the format "BFPO NNN" or "BFPO c/o NNN", where NNN is 1 to 4 numerical digits. A regular expression to implement the BS 7666 rules:^[49]

(GIR 0AA)|((([A-Z-[QVX]][0-9][0-9]?)|(([A-Z-[QVX]][A-Z-[IJZ]][0-9][0-9]?)|(([A-Z-[QVX]][0-9][A-HJKSTUW])|([A-Z-[QVX]][A-Z-[IJZ]][0-9][ABEHMNPRVWXY])))) [0-9][A-Z-[CIKMOV]]{2})

An alternative short regular expression from BS7666 Schema is:

[A-Z]{1,2}[0-9R][0-9A-Z]? [0-9][ABD-HJLNP-UW-Z]{2}

The above expressions fail to exclude many non-existent area codes (such as A, AA, Z and ZY). A more refined regex, which excludes all invalid areas and some invalid districts, is:

((A[BL]|B[ABDHLNRST]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[HNX]?|F[KY]|G[LUY]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGKLMNOPRSTY]?|T[ADFNQRSW]|UB|W[ADFNRSV]|YO|ZE)[1-9]?[0-9]|([E|N|NW|SE|SW|W]1|EC[1-4]|WC[12])[A-HJKMNPR-Y]|[SW|W]([1-9][0-9]|[2-9])|EC[1-9][0-9]) [0-9][ABD-HJLNP-UW-Z]{2}

Mailsort

As the format of the code does not make it easy to group items together into operationally significant blocks, it has 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'.^[50] Mail users who can deliver mail to the post office split up by Mailsort code receive discounts but [bulk] delivery by postcode provides no such incentive.

Non-postal uses and economic aspects

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

• 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 streetmap software, 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^[51].
• Postcodes can be used with satellite navigation systems to navigate to an address by street number and postcode.
• Postcodes are used by non-life insurance companies to assess premiums for motoring/business/domestic policy premiums.
• Postcodes are used by life insurance companies and pension funds to assess longevity for pricing and reserving.^[52]

Availability

The availability of postcode information has significant economic advantages. As of October 2009^[update] the Royal Mail was licensing use of the postcode database for a charge of about £4000 per year.^[51] Following a Government consultation,^[53] on 1 April 2010 Ordnance Survey released co-ordinate data for all Great Britain postcodes (but not their address elements) for re-use free of charge under an attribution-only license, as part of OS OpenData.

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

References

External links

• Royal Mail has two ways to search addresses or postcodes from its website, allowing up to 15 searches a day free of charge (these cover nearly all UK and crown dependency postcodes but not the non-geographic BX postcode area nor "GIR 0AA"):
  • Royal Mail's Online Postcode Finder (returns the postcodes covering a specified street, or the postcode for a specified postal address)
  • Royal Mail's Online Address Finder (returns the postal addresses covered by a specified postcode)
• Beacon Dodsworth: Postcode updates
• Evox Facilities: Explanation of the UK Postcode System
• Lodge Models: UK postcode map
• UK Government Data Standards Catalogue: Postcodes
• Universal Postal Union: United Kingdom of Great Britain and Northern Ireland
• Strowger Net: Postcodes of the UK