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Explanation of the UK Postcode System

Postcodes were introduced over 50 years ago to speed mail distribution. Though serving this initial purpose, uses of the system have developed into a widely applied research, analysis, planning, routing and management tool. The structure of the postcode is explained below.

Postcodes are alphanumeric strings of 5 to 7 characters length, split by a single space into an Outward and Inward code, each of which comprise two distinct parts. Including the single character space a full Postcode is a minimum of 6 characters and a maximum of 8. There are 1.7 million unique full postcodes. Example: postcode of the Brighton Royal Pavilion is "BN1 1EE":

  • the character string before the space is the OUTWARD code eg "BN" = Area, "1" = District, hence District Level code = "BN1"
  • the character string after the space is the INWARD code eg "1" = Sector, hence Sector Level code = "BN1 1" and "EE" = Walk
  • there are 124 AREA codes, 116 use two alpha characters, 8 use a single alpha character; Area Listing : Area Map
  • there are appx 3,000 District level codes. The District element is 1 or 2 numerics (in parts of London, it can also be numeric and alpha)
  • there are appx 11,100 Sector level codes. The Sector element is always a single numeric
  • the final 2 alpha characters, known as the Walk, refer to groups of houses, in some cases individual premises or PO Boxes.
  • full Postcodes comprise Area District Sector Walk, result is 4 code patterns of: "AD SWW", "ADD SWW", "AAD SWW" or "AADD SWW"
  • The Outward code can vary from 2~4 characters: "AD ~", "ADD ~", "AAD ~" or "AADD ~"
  • The Inward code is fixed at 3 characters: "~ SWW", being a single numeric then 2 alpha chrs.
  • The alpha characters in a code are always shown in upper case. Note: though used in the Outward code, the letters C, I, K, M, O and V are not used in the Inward code.

There are appx 1500 PostTowns (being Cities or major towns) used in UK addressing as an integral part of a correct address; hence these are frequently used as reference point in geographic analysis. In our "BN1 1EE" example, "BN" defines the postal Area as Brighton, not that the PostTown is Brighton. There are 18 PostTowns in use in the "BN" Area, ranging from Arundel to Worthing; as it happens the PostTown for our "BN1 1EE" example is Brighton. Most Areas comprise many PostTowns, the noticeable exception, where the inverse applies, is for the 8 London Areas (i.e. E, EC, N, NW, SE, SW, W and WC) where London is the PostTown.

Nearly all the UK 124 postcode Areas are known by one of their PostTowns (eg AB=Aberdeen, B=Birmingham, etc); though this assists in interpreting them, the actual coverage of postal Areas can often surprise those who interpret it literally. This is illustrated well by the "AB" Area, but equally applies to many others as shown in our PostTowns by Area : Area Map.

Despite increase in the number of UK "Counties" following administrative/political/boundary changes, the number of Postcode Areas still exceeds that of counties, 124 to 110 respectively (latter was previously 76). District level codes exceed Counties by a factor of more than 27x, which increases at Sector level code to 100x. The constituents of the postcode system provide increasingly accurate and nationally accepted means of identifying geographic location, which is why it has become so widely used outside of mail delivery. Homepage

This link provides background to recent developments concerning the postcode and brief outline of our experience and expertise of UK postcodes and addressing.


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