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Postcodes

Postcodes are stored in Code-Point as alphanumerical values with a maximum length of eight characters (the number of characters in a postcode can vary from six to eight). A postcode contains two parts:

• The outcode (also called outward code): The first two to four characters of the postcode, constituting the postcode area and the postcode district. It is the part of the postcode that enables mail to be sent from the accepting office to the correct area for delivery.

• The incode (also called inward code): The last three characters of the postcode, constituting the postcode sector and the postcode unit. It is used to sort mail at the local delivery office.

For example:

Outcode		Incode
NW	6	4	DP
Area	District	Sector	Unit

When used in an address, the incode is always separated from the outcode by a single space e.g NW6 4DP. The single space counts as a character.

The following table provides a list of the valid formats of postcodes. In the first two columns, an A indicates an alphabetic character, while an N indicates a numeric character. The last column shows how many characters each example postcode contains.

Outcode	Incode	Example postcode	Number of characters in the example postcode
AN	NAA	M2 5BQ	6
ANN	NAA	M34 3AB	7
AAN	NAA	DN5 7XY	7
AANN	NAA	DN16 9AA	8
ANA	NAA	W1A 4WW	7
AANA	NAA	EC1A 1HQ	8

Postcode position

Each postcode unit will be allocated a National Grid reference of a point that falls within the notional extent of the postcode unit, given as an Easting and Northing. This point is the co-ordinate of the nearest delivery point to the calculated mean position of the delivery points in the postcode unit. These coordinates are provided to a resolution of 1 metre.

Where several postcode units apply to one surveyed position (for example, a block of flats or offices), there is an identical location for each. However, there may be instances where the location is imprecise or approximate, due to the manual allocation by Royal Mail of a postcode outside the recognised geographical extent of that postcode. There may also be a small number of instances where coordinates cannot be allocated.

The accuracy of each postcode unit coordinate pair is defined by the Positional Quality indicator (PQI), which provides a quality statement for the position of that Code-Point record. There are seven PQI values for the positional quality. A lower positional quality indicator will be allocated to postcode units awaiting a surveyed position, or which relate to addresses that do not have a surveyed position. Full details of the PQI can be found in the technical specification.

Coordinate reference systems

The coordinates for postcodes in Great Britain (England, Wales, and Scotland) are provided in British National Grid (BNG). BNG uses the OSGB36 (EPSG 27700) geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using Easting and Northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not specify a vertical (height) reference system.

The coordinates for postcodes in Northern Ireland (BT postcodes) are provided in the Irish National Grid. When loading the BT postcodes into a GIS, you should select the Irish Grid (EPSG: 29902) for the correct projection. If you do not apply the Irish projection, the BT centroid points will not be correctly positioned. For additional information relating to the Irish Grid Reference System, see the Irish Grid reference document here (PDF download).