Postcodes in The United Kingdom - Operation and Application - Format

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

This can be generalised as: (one or two letters)(number between 0 and 99)(null or one letter)(space)(single digit)(two letters)

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, 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, BS, CM, CR, FY, HA, PR, SL, SS; BS is the only area to have both a district 0 and a district 10.
    • 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, which excludes 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, which excludes 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.