A UK postcode contains five to seven letters and numbers and identifies four levels of geographic area. Royal Mail uses this system to sort and deliver mail. Each postcode represents a group of delivery points, such as homes, businesses or post boxes.

A postcode has two parts, separated by a space:

• The outward code identifies the postcode area and district, directing mail to the correct local area.
• The inward code helps the local delivery office sort the mail. It contains one number identifying the postcode sector, followed by two letters identifying a specific group of properties—or a single large organisation.

For example, in PO1 3AX:

• PO is the Portsmouth postcode area. The UK has 124 postcode areas including RG (Reading), BR (Bristol), LS (Leeds).
• PO1 is a district within the Portsmouth postcode area. The UK has approximately 2,900 postcode districts.
• PO1 3 is the postcode sector. The UK has approximately 9,650 postcode sectors.
• PO1 3AX is the complete postcode. The final two letters, AX, identify the postcode unit, covering one or more small delivery points or a single large user.

There are over 1.78 million active postcodes across the UK.