Postcodes in the United Kingdom

Postal codes used in the United Kingdom, British Overseas Territories and Crown dependencies are known as postcodes (originally, postal codes). They are alphanumeric (the UK is one of only 11 countries or territories to use alphanumeric codes out of the 160 postcode-using members of the ICU).

Postcodes are essentially a location specific routing instruction. The codes designate 121 geographic postcode areas which are mnemonically named after the area's major post town (such as TR for TRuro) although some are named after smaller towns or regional areas. Each postcode area is subdivided into districts, which in turn are subdivided into sectors. A full postcode identifies a group of addresses (typically around ten) or a major delivery point.

The current postcode system evolved from named postal districts introduced in London in 1857 and gradually in other large cities from the 1860s onwards. Districts in London were then subdivided in 1917, with each allocated a distinct number. This was extended to other cities by 1934. The current system (building on the earlier postal districts) was devised by the General Post Office (Royal Mail) and was adopted nationally in stages between 1959 and 1974.

The system was designed with a single objective, to aid (the now) Royal Mail in mechanised sorting of mail for delivery. Since its roll-out, changes in business needs and new technology have meant that postcodes are now also used in many differing ways: they serve as aggregation units in census enumeration; designate destinations in route planning software, and help calculate insurance premiums.

Royal Mail is statutorily responsible for allocation of all postcodes and it continuously maintains the Postcode Address File (PAF) database, which stores address data for around 32 million addresses and around 1.8m postcodes. The PAF (and access to its data) is overseen by the independent PAF Advisory Board and Ofcom, which regulates communication services in the UK.