Royal Mail maintains a UK-wide system of postcodes to identify postal delivery areas.
Most people know their postcode, so we are able to use this as the main geographic reference when collecting data.
This reference can be related to any geographic unit used for statistical production, such as a local authority district or electoral ward.
So postal geography is very valuable.
Postcodes are alphanumeric references comprising an outward code of 2–4 characters and an inward code of three characters. For example:
Postcode structure 1
|Outward Code||Inward Code|
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The postcode is structured hierarchically, supporting four levels of geographic unit:
Postcode structure 2
|Example||Geographic Unit||Number in UK|
|PO15 5||Postcode Sector||12,381|
|PO15 5RR||Unit Postcode||Approximately 1.75 million (Live)|
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As at November 2014, these 1.75 million postcodes comprise approximately 1.8 million small user and 0.7 million large user postcodes, including the Channel Islands and the Isle of Man (see below).
Unit postcodes are the base unit of postal geography and fall into two types:
Large user postcodes: allocated to single addresses receiving at least 500 mail items per day (e.g. 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 that the public and businesses are familiar with.
Linking postal geographies to other geographic units is far from straightforward though, as:
Postcode boundaries do not align with other geographic boundaries. If a unit postcode straddles a ward (or other) boundary, you have to decide which ward to allocate the data to. Our 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 ‘other’ 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 “beginner’s 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 postcodes 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.