UK postal codes are known as postcodes. They are alphanumeric. These codes were introduced by the Royal Mail over a fifteen year period from 1959 to 1974. 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 below.
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. Mail users who can deliver mail to the post office sorted by mailsort code receive discounts, whilst delivery by postcode provides no such incentive.
Table of contents
The format of UK postcodes is generally:
- LD DLL
- LLD DLL
- LDD DLL
- LLDD DLL
- LLDL DLL
- LDL DLL
where L signifies a letter and D 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.
The part of the code before the space is the outward code used to direct mail from one sorting office to the destination sorting office (the alphabetic part identifying one of 121 postal districts), while the part after the space is the inward code used to sort the mail into individual postmen's delivery rounds, each separate code usually identifying the address to within 80 properties, although large businesses may have a unique code.
The letters in the outward code can only be one of a set of known combinations, which usually gives some clue to its geographical location (but see London below). For example, BS indicates Bristol and G indicates Glasgow; see List of postal codes in the United Kingdom for a full list. BT indicates Belfast, but actually specifies anywhere within Northern Ireland. The letters in the inward code, however, are restricted to the set ABDEFGHJLNPQRSTUWXYZ (so cannot be one of CIKMOV), which generally do not resemble digits or each other when hand-written.
There is one exception (other than the overseas territories) to this format; the postcode for the formerly Post Office-owned Girobank is GIR 0AA.
Main article: London postal district.
- In central London, WC and EC (West Central and East Central)
- In the rest of London, N, NW, SW, SE, W and E.
Note that London postal districts rarely coincide with the boundaries of London boroughs (even the old, smaller boroughs). The numbering system also 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 closest to the centre) they were numbered alphabetically by the name of the main sorting office.
In part of outer London the postcodes follow the more conventional pattern of postcodes deriving from the main sorting office. To confuse matters further many of these parts of London have postal addresses that use the traditional county boundaries — for example postal addresses in Sutton traditionally read "Sutton, Surrey" and not "Sutton, London".
Several postcode areas cross administrative boundaries, covering parts of neighbouring administrative areas as well as areas in Greater London. For example, KT (Kingston upon Thames), TW (Twickenham), SM (Sutton) and CR (Croydon) cover parts of Surrey, while DA (Dartford) and BR (Bromley) cover parts of Kent, and RM (Romford) and IG (Ilford) covers part of Essex.
A further complication is that in some of the most central London areas, a further gradation has been necessary to produce enough postcodes, giving unusual 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
In the 1930s the residents of some areas on the fringes of the London postal area were asked if they would prefer to receive their post from London or another post town. One of the factors which persuaded communities, e.g. Kingston Vale in Kingston upon Thames, to become part of London postal district SW15 rather than receive their post from Kingston was that London had two postal deliveries a day. With the decline in the London postal delivery service from 2003 to one post a day this no longer applies and so there have been motions by residents to reconsider their affiliation. Residents of Ilford and Windsor have also suggested the possibility of altering their postcode but have been told by Royal Mail that there is "virtually no hope" of changing their postcode .
The disadvantage of a London postal district is that it often carries a higher household and car insurance premium. Other disadvantages stem from the fact that many organisations adopt the postcode database for marketing, distribution and servicing purposes: "the postcode lottery". Thus health and ambulance services, telephone directories, shop deliveries, etc. are decided by postcode rather than by more rational principles. Still other disadvantages are the association of a postcode with a postal address. For instance Denham, in Buckinghamshire, has its letters sorted in Uxbridge, has a UB9 postcode and is described in the postcode database as 'Denham, UXBRIDGE, Middlesex'. Letters are still delivered by postmen, certainly, but delivery drivers can be misled into seeking a house in the wrong town which may be quite a distance from the intended delivery point.
Other cities' postcodes
Until the 1960s, cities such as Belfast, Birmingham, Edinburgh, Glasgow, Leeds, Liverpool, Manchester, Newcastle upon Tyne and Sheffield were divided into different postal districts, each with a number, 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 would start with L8, followed by the rest of the postcode. A similar system is still used in Dublin in the Republic of Ireland, the Dublin postal districts.
The consequence of the complexity outlined above is that for almost every rule concerning UK postcodes, an exception can be found which breaks that rule. 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. 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.
It is possible to validate the format of a postcode using the following rules:
- The postcode must be of 6, 7, or 8 characters in length.
- The outward code (the set of characters to the left of the space) must be 2, 3 or 4 characters in length
- The first character of the outward code must be alphabetic.
- The inward code (the set of characters to the right of the space) must always be 3 characters in length.
- The first character of the inward code must be numeric.
- The second and third characters of the inward code must be alphabetic.
A regular expression that validates the format rather loosely is
All valid postcodes (except GIR 0AA) will match this, but several invalid postcodes will also match.
The PAF is commercially licensable 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 http://www.streetmap.co.uk for an example of this in practice.
The Channel Islands (Jersey and Guernsey) and the Isle of Man established their own separate postal administrations 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.
Some of the UK's overseas territories have their own postcodes:
- Falkland Islands: FIQQ 1ZZ
- South Georgia and the South Sandwich Islands: SIQQ 1ZZ
- British Antarctic Territory: BIQQ 1ZZ
- St Helena: STHL 1ZZ
- Ascension Island: ASCN 1ZZ
Unlike UK postcodes, these are used for all addresses in those territories. The reason why they were introduced is because mail was often sent to the wrong place, e.g: St Helena to St Helens in England, Falklands to Falkirk in Scotland. 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.
A Royal Mail Information Sheet (see External links below) describes how postcode trials were conducted in Norwich and Croydon. (It is possible that these two towns were selected because they had the letter O in their names at positions where the designers were thinking of using a digit. For example they may have had in mind for Norwich the Canadian format of LDL DLD.) Norwich was completely re-coded but the scheme tested in Croydon was sufficiently close to the final design for it to be retained.
Another early adopter of postcodes was Newport. Here, Newport itself was allocated NPT, in a similar way to Norwich and Croydon, with the surrounding towns allocated NP1-NP8. This lasted into the mid '80s when for operational reasons (NPT being non-standard, and too similar to NP7) it was finally recoded.
The legacy of the Croydon trial can still be seen today:
- CR0 was the only postal district with a zero in that position when all others started at 1. This caused one of the PAF (see above) software products produced by the Royal Mail themselves to mis-behave slightly! Subsequently, the "zeroth" district has been used in some other postcode areas.
- A separate postal "district", CR9 is used for large users. 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.)
- ACORN (geodemography)
- British Forces Post Office
- Post town
- UK topics
- ZIP Codes – The US version
- List of postal codes for worldwide postal code definitions.
- More detailed explanation of the postcode system
- British postal codes – this PDF document from the Universal Postal Union explains the system and shows all allowable UK postcode formats
- Royal Mail's Information Sheet on the history of Postcodes – PDF