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Explanation of the UK Postcode System

 

Postcodes were introduced over 40 years ago to speed mail distribution. Though serving this initial purpose, use of the system has developed into a widely applied research, analysis, planning 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. Hence 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. Using PO1 1AF the postcode of the Royal Mails Address Management Unit, as an example:

 

  • the character string before the space is the OUTWARD code eg “PO” = Area, “1” = District, hence District Level code = “PO1”
  • the character string after the space is the INWARD code eg 1 = Sector, hence Sector Level code = “PO1 1” and AF = Walk
  • there are 124 AREA codes, 116 use two alpha characters, 8 use a single alpha character; Area Listing : Area Map
  • there are appx 2,980 District level codes. The District element is 1 or 2 numerics (except in parts of London where numeric and alpha)
  • there are appx 10,600 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, hence 4 patterns: “AD SWW”, “ADD SWW”, “AAD SWW” or “AADD SWW”
  • The Outward code can vary from 2~4 characters length, “AD ~”, “ADD ~”, “AAD ~” or “AADD ~”
  • The Inward code is fixed 3 character length i.e.”~ SWW”, comprising a single numeric followed by 2 alpha characters
  • All alpha characters in a Postcode are shown in upper case - note the letters C, I, K, M, O and V are not used in the Inward code

 

There are 1500 PostTowns (city or major town) used by the Royal Mail and integral from their view to correct addressing; hence these are frequently used as reference point in geographic analysis. In our PO1 1AF example, PO defines the postcode Area as Portsmouth not that the Post Town is Portsmouth; there are 24 Post Towns in use in the PO Area, ranging from Bembridge to Yarmouth; as it happens the Post Town for the PO1 1AF example is Portsmouth. Most postcode Areas comprise many Post Towns, the noticeable exception where the inverse applies is for the 8 London postcode Areas (i.e E, EC, N, NW, SE, SW, W and WC) where London is the PostTown.

 

Nearly all 124 Postcode Areas are known by one of their Post Towns (eg AB=Aberdeen, B=Birmingham, etc); though assisting in interpreting them, the coverage of postal Areas can often surprise those who interpret this literally. This is illustrated by the AB postal Area, but equally applies to many others, in our PostTowns by Area : Area Map.

 

Despite increase in the number of UK "Counties" following recent administrative/political/boundary changes, the number of Postcode Areas still exceeds that of counties, 124 to 110 respectively (latter was previously 76). District level postcodes exceed Counties by a factor of more than 25x, which increases to 95x at Sector level postcode. 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

 

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