In this blog, we reveal some interesting facts you probably didn't know about postcodes.

A red poster showing a pile of letters with the headline 'Use the Postcode. You're not properly addressed without it!'

‘Use the postcode’ poster, 1979 (POST 110/0162)

1. Old and new

Every month around 3,000 new postcodes are created and on average about 120 are deleted.

2. The smallest and biggest

WC in London is the smallest postcode area in Britain, covering just 1 square mile. The largess is IV, Inverness, which covers 6,243 square miles.

3. The number of postcodes

1.8 million postcodes are in use in the UK and these cover more than 30 million individual addresses.

4. Dividing the country

The UK is divided up into 124 postcode areas. These are split into 2,979 districts, 11,232 sectors and around 1.8 million units.

5. Making of the postcode

In the UK the postcodes are properly written in two parts. The letters and numbers describe different parts of the address. The first part is known as the ‘outcode’ and the second part is the ‘incode’. The outcode tells Royal Mail which part of the country to take the letter or parcel to. The incode tells them which local postal route the item is covered by.

6. Sorting of the past

Before postcodes and mechanised sorting, a Victorian postal worker could sort letters into a sorting frame with 48 boxes at speeds of up to 30 letters per minute.

A line of men in black uniform jackets, seated behind a long row of shelves.

Postal Staff in a Sorting Office, Lantern Slide, c.1903 (2011-0187/49)

7. Postal districts

District codes were the first step in the story of postcodes. London was divided up into coded districts in 1858, followed by other large towns and cities in the 1860s. Rowland Hill suggested the idea to speed up sorting of mail and help solve the problem of duplicate street names.

A large map with the heading 'Map of London Postal Districts and Sub-Districts'.

‘Map of London Postal Districts and Sub-Districts’ c.1858 (POST 21/55)

8. The first sorting machine

Transorma was the first sorting machine trialled by the Post Office. It was built in the Netherlands and tested at Brighton Post Office from 1935 but wasn’t a success.

A large room with machinery in the middle.

Transorma, a view of tray conveyors at the Brighton Sorting Office, 1935 (POST 118/16241)


ELSIE was the first successful sorting machine built in Britain, designed by the engineers of the world-famous Post Office Research Station at Dollis Hill, in north-west London. It combined the mechanical wizardry of the Dutch Transorma with the electrical circuitry of the secret Colossus computer.

10. Guessing game

But what happens if you forget to include the postcode when you address a letter? Or your postcode can’t be read by the machines? Royal Mail staff will try to deduce the destination themselves. They can do this onsite or a via live video link through cameras connected to the sorting machines. If they really can’t work out the address and there are no details to help return the letter it will be sent to the National Returns Centre in Belfast. This vast warehouse contains up to 20 million undelivered letters and parcels.

11. Special postcodes

Postcodes are allocated by Royal Mail’s Address Management Unit and cannot be purchased or specified by the recipient. However, Royal Mail sometimes assigns special postcodes to high-profile organisations that contains clues as to the owner. For example, the US Embassy is at SW11 7US or the BT’s correspondence address in Durham postcode area at DH98 1BT.

12. Fictional postcodes

Coronation Street, a fictional street in Manchester, had its own postcode! M10 9KC.
Even Father Christmas has a postcode, XM4 5HQ, and he gets letters every year in the 1000s.