A British postcode looks like SW1A 1AA or M1 1AE, and it identifies not a town or a district but a cluster of around fifteen addresses. That precision is the key to everything else about it.
Reading one
A postcode has two halves separated by a space.
The outward code — SW1A — gets the letter to the right sorting office:
- One or two letters for the postcode area: SW for south-west London, M for Manchester, EH for Edinburgh, CF for Cardiff.
- Then a number, and occasionally a letter, for the district.
The inward code — 1AA — gets it to the door:
- A number for the sector, a group of streets.
- Two letters for the unit, a specific side of a street or a run of houses.
There are around 1.8 million live postcodes covering roughly 30 million addresses.
Why they are so precise
The system was built for mechanised sorting. Trials began in Norwich in 1959, and nationwide rollout completed in 1974. Because the last two letters narrow the address to a handful of properties, a postcode plus a house number is a complete address — and Royal Mail will deliver on that alone.
This is why British satnavs, delivery apps and forms all ask for a postcode first and then simply offer you a list of the houses in it.
What else they are used for
Far more than post, and this is where it becomes consequential:
- Insurance premiums — car and home insurance is priced by postcode, and moving a few streets can change the quote substantially.
- School catchment areas — which drives house prices around good schools to a degree that is a persistent national grievance.
- Deliveries and takeaways — whether a service covers you at all.
- Statistics and funding — deprivation indices, census output and NHS resource allocation are all built on postcode geography.
- NHS treatment availability — giving rise to the phrase “postcode lottery”, used constantly in British political argument to describe services that vary by where you happen to live.
The famous ones
- SW1A 1AA — Buckingham Palace.
- SW1A 2AA — 10 Downing Street.
- SW1A 0AA — the House of Commons.
- E20 — invented for EastEnders because it did not exist, then genuinely allocated to the Olympic Park in 2011.
The oddities
A few postcodes are non-geographic. GIR 0AA belonged to Girobank in Bootle. Some large organisations receiving very heavy post have their own — the BBC, government departments and certain charities — so that a single building has a code to itself.
Ireland, incidentally, had no postcode system at all until Eircode was introduced in 2015, which visitors from Britain found difficult to believe.