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Postal Geography

Royal Mail maintains a UK-wide system of postcodes to identify postal delivery areas.

Most people know their postcode so ONS are able to use this as their main geographic reference when collecting data.

This reference can be related to any geographic unit used for statistical production, such as a district or electoral ward.

Postal geography is thus very valuable. 

Postcode structure

Postcodes are alphanumeric references comprising an outward code of 2-4 characters and an inward code of three characters. For example:

PO16 7DZ
Outward Code Inward Code

The postcode is structured hierarchically, supporting four levels of geographic unit:

Example Geographic Unit Number in UK
PO Postcode Area 124
PO16 Postcode District


PO16  7 Postcode Sector


PO16 7DZ Unit Postcode

1.75 million approx (Live)

As at August 2011, these 1.75 million postcodes comprise approximately 1.6 million small user and 0.14 million large user postcodes, including the Channel Islands and the Isle of Man (see below).

Unit postcodes

Unit postcodes are the base unit of postal geography and fall into two types:

  • Large user postcodes: allocated to single addresses receiving at least 1,000 mail items per day (eg business addresses)

  • Small user postcodes: collections of (usually) adjacent addresses. A single small user postcode may contain up to 100 addresses, but 15 is a more typical number

Note: It is possible for large buildings with many separate delivery points (for example, a tower block) to have more than one unit postcode within the building.

Limitations of using postcodes as a geographic reference

As indicated, postcodes form a compact geographic reference with which the public and businesses are familiar.

Linking postal geographies to other geographic units is far from straightforward though as:

  • Postcode boundaries are not contiguous with other geographic boundaries. If a unit postcode straddles a ward (or higher level) boundary, you have to decide to which ward to allocate the data. ONS Geography's postcode directories take the grid reference of the postcode centroid and match this up to digital administrative boundaries. However, some addresses (and therefore data) will still inevitably be allocated to the wrong area. Note though that this problem will be reduced in future with the move towards using address-based rather than postcode-based grid references. See our Beginners Guide to Geographic Referencing for more details.

  • Postcode boundaries are subject to continuous change due to new addresses, single addresses acquiring large user postcodes as mail volume increases, and the need to restrict the number of addresses per unit to less than 100. Areas can also be recoded and codes can be re-used in a different place after just two years. Continuous monitoring is therefore required to avoid data misallocation.

Information on postcode recoding

Details of these larger reorganisations are provided in the Royal Mail's 'Postcode Update' series.

The most recent Postcode Updates, together with summary information on major postcode changes since 1990, are available on the Royal Mail website.

Content from the Office for National Statistics.
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