Postcodes are the bedrock of marketing databases, demographics and profiling in the UK.

But it was never supposed to be that way.

The postcode system was developed by, and is owned by, Royal Mail and it was simply set up to help Royal Mail deliver letters and parcels more easily.

Many years later, Royal Mail discovered it had a powerful and potentially lucrative tool that could help marketeers. But it isn’t primarily there to help anyone else and sometimes it is a system that can bite other users!

The postcode system knows no boundaries.

County, town and other administrative boundaries only follow postcode boundaries by accident. Counties are not considered relevant in addresses and whole counties may be covered by one town eg most of Suffolk has a postcode starting ‘IP’ for Ipswich. Even parts of Scotland and Wales have been considered to be in England in the past.

This of course has not been helped by frequent local government reorganisation, but even without local government reorganisation postcode boundaries tend only to change when there is a significant increase or reduction of population, such as the creation of a new postcode E20 for the Olympic Park in Stratford, London.

The anatomy of a postcode

This table is best viewed on a computer/laptop.

Outward code

Inward code


Post area

Postcode district


Postcode sector

Postcode unit

Outside London One or two letters representing the post town, eg L for Liverpool, IP for Ipswich One or two digits, occasionally with an additional letter at the end 1,2,3,4,5,6,7,8,9,0 [Note 1] Two alpha characters
Example:IP1 2DE IP 1 2 DE
London One or two letters mainly based on points of the compass. [Note 2] One or two digits, occasionally with an additional letter at the end 1,2,3,4,5,6,7,8,9,0 [Note 1]

Two alpha

EC 1A 2 BC


Note 1 Postcode sector

  • 0 is considered to be ’10’ not zero.
  • Not all digits are always used for every district. Gaps may
    appear in any position eg you may have 1,2,4,7,8

Note 2 London

One or two letters mainly based on points of the compass:N,E,SE,SW,W,NW

  • but with some missing (NE, S)
  • and some additional: plus EC (East Central)
    and WC (West Central).
  • plus other postcodes around the edges of Greater London
    based on the original post towns:EN (Enfield), IG (Ilford), RM (Romford), DA (Dartford), BR (Bromley), CR (Croydon), SM (Sutton), KT (Kingston), TW (Twickenham), UB (Uxbridge), HA (Harrow), WD (Watford)
  • The Queen Elizabeth Olympic Park has its own unique E20 postcode, with three sectors: E20 1E20 2 and E20 3E20 is also the fictional postcode for Walford in the BBC’s Eastenders programme.

Other cities

Some other cities have their own variations on the postcode system as they historically had a postcode system in palce before Roayl Mail introduced a standardised format, including eg: Liverpool, Manchester and Glasgow.

Large organisations

Many large organisations have their own unique postcode, eg: The House of Commons SW1A 0AA, or the DVLA which has different postcodes for different services eg SA6 7JL, SA99 1AR.

Post Office Address File (PAF file)

The Post Office Address file, usually referred to as the PAF® file is Royal Mail’s database of all known addresses and postcodes in the United Kingdom (England, Scotland, Wales, Northern Ireland), Jersey, Guernsey, and the Isle of Man and holds over 28 million addresses and 1.75 million postcodes of addresses that have a letterbox. A few Northern Ireland addresses are not included, nor are British Forces Post Office addresses.

For more details about the PAF® file see: Royal Mail AddressNow Data

Clear addressing

Constructing your contact database in a file format that is compatible with Royal Mail’s PAF® file will improve your chances of mail being delivered correctly but will also become important and save money if you use Mailsort (a Royal Mail postage discount scheme) or are trying to import data eg from a list broker or demographic profiling system or you are exporting data, for example for a mailing house.

Even if you’re only starting off with data on a simple spreadsheet, following a clearly worked out field structure will ensure you can make use of sort, filter, pivot table and other facilities without data getting scrambled or lost and thatb you can meet data protection standards. Contact me if you need help with this.

Royal Mail provides simple guidance for correct addressing, plus a substantial guide for programmers.

More information from Royal Mail: Royal Mail Clear Addressing


Code-Point is a project between Ordnance Survey and Royal Mail which maps postcodes with geographic locations.

There are two versions, Code-Point Open which is free and Code-Point which has more features aimed at business and is available at a charge.

More information: Ordnance Survey Code-Point