The UK Postcode System Explained: How Postcodes Work

The Structure of a UK Postcode

Every UK postcode has two halves, separated by a space. Take SW1A 1AA (Buckingham Palace) as an example:

• SW1A is the outward code. It tells the sorting office which area and district the mail is going to.
• 1AA is the inward code. It narrows it down to a specific group of addresses (a "postcode unit").

The outward code is further broken down:

• SW is the postcode area (South West London). There are 124 postcode areas in the UK, each identified by one or two letters.
• 1A is the postcode district. There are roughly 3,000 districts across the country.

And the inward code:

• 1 is the postcode sector.
• AA is the postcode unit - the final two letters that identify a specific group of roughly 15 addresses.

How Many Postcodes Are There?

There are about 1.8 million active postcodes in the UK, covering around 31 million addresses. New postcodes are created regularly as housing developments are built, and old ones are retired when buildings are demolished or areas are redeveloped.

Royal Mail maintains the Postcode Address File (PAF), which is the definitive database of all UK postcodes and their associated addresses. It's updated regularly and is used by every business and service that relies on postcode data.

What the Letters Mean

The area letters usually relate to the main town or city that the sorting office serves:

• B - Birmingham
• M - Manchester
• LS - Leeds
• EH - Edinburgh
• CF - Cardiff
• BT - Belfast

London has its own system based on compass directions: N (North), E (East), SE (South East), SW (South West), W (West), NW (North West), EC (East Central), and WC (West Central).

Postcodes and Geography

Postcodes don't follow council boundaries, constituency borders, or any other administrative geography. They follow Royal Mail delivery routes. This means a single postcode area can span multiple local authorities, and a single street can sometimes fall in different council areas despite sharing a postcode.

This causes real problems. Your postcode determines things like which council tax band you're quoted, which school catchment area lookup tools suggest, and which broadband providers appear when you check availability. If your postcode sits on a boundary, you might get inaccurate results.

Unusual Postcodes

Some postcodes don't fit the normal pattern:

• GIR 0AA was the old postcode for the National Girobank in Bootle. It's no longer in active use but still appears in some databases.
• BF1 (and BFPO) - British Forces Post Office postcodes for military mail. BF1 postcodes were introduced to give forces addresses a standard UK postcode format.
• Santa Claus gets his own postcode: XM4 5HQ (formerly SAN TA1). Royal Mail processes about 750,000 letters to Santa each year.
• Single-address postcodes. Some large buildings or organisations have their own unique postcode. Buckingham Palace (SW1A 1AA), the Houses of Parliament (SW1A 0AA), and large businesses with heavy mail volumes often get their own.

Why Nearby Addresses Can Have Very Different Postcodes

Because postcodes follow delivery routes rather than geography, two houses on opposite sides of a street can have completely different outward codes. This is especially common on the boundaries of postcode areas - for example, where a Manchester (M) postcode meets a Stockport (SK) postcode.

It also means that your nearest postcode geographically isn't always obvious. Our Nearest Postcodes tool is useful for finding which postcodes are physically closest to a given location, regardless of which area or district they belong to.

Postcodes and Property Prices

Postcodes have a real impact on property values. A "good" postcode can add thousands to a house price, even when the actual location is almost identical to a neighbouring "bad" one. Estate agents know this and will always highlight a desirable postcode in their listings.

The effect is most pronounced at the boundaries between affluent and less affluent areas. A house on the "right" side of a postcode boundary can be worth 10-15% more than an identical house 200 metres away with a different postcode.

How New Postcodes Are Created

When a new housing estate is built, the developer works with Royal Mail to assign postcodes. Royal Mail follows a set of rules: the postcode must fit the existing area structure, the outward code is determined by the sorting office that will handle the mail, and the inward code is assigned sequentially within the district.

Certain letter combinations are avoided in the inward code to prevent confusion: the letters C, I, K, M, O, and V are never used in the final two positions. This is because they could be confused with other characters (O looks like 0, I looks like 1, etc.).

Look Up Postcodes Near Any Location

Our Nearest Postcodes tool lets you enter any UK postcode and instantly see the closest postcodes ranked by distance. It's useful for finding alternative postcodes when yours doesn't return the right results in a search, or for understanding the postcode geography of an area you're moving to.