How the tool finds your postcode
Your browser asks your device for its position, which on a phone usually comes from GPS and Wi-Fi and is accurate to a few meters outdoors. In Great Britain and Northern Ireland, the tool then asks postcodes.io, a free service built on the Office for National Statistics Postcode Directory, for the postcode whose centre point is nearest to you. Every UK postcode has one representative point placed among its addresses. At the same time, OpenStreetMap’s Nominatim service returns the nearest street address, so you can check the result against your own front door.
Outside the UK there is no equivalent open, official directory for most countries, so the postcode comes from the address OpenStreetMap holds for that spot. Either way, only the coordinates are sent to those two services for the lookup; nothing is stored by this site. See the Privacy Policy for details.
How a UK postcode is built
Royal Mail’s address file holds around 1.8 million postcodes. Each one covers about 15 addresses on average, and an ordinary postcode never covers more than 100 delivery points. Every postcode narrows down in four steps, which the tool shows under each result. Taking SW1A 2AA:
| Part | Example | What it identifies |
|---|---|---|
| Area | SW | One or two letters for a large region, usually named after a main town or, in London, a compass direction (South Western). |
| District | SW1A | The whole first half, called the outward code, which takes mail to the right delivery office. |
| Sector | SW1A 2 | The district plus the first digit of the second half: a few streets or a neighbourhood. |
| Unit | SW1A 2AA | The full postcode: one side of a street, a block of flats, or a single large organisation. |
The second half, the inward code, is always a digit followed by two letters, which is why a missing space rarely matters but a swapped letter does. Organisations that receive a lot of post can have a postcode of their own.
Why the postcode shown might not be yours
- You are near a postcode boundary. Postcode areas follow streets and buildings, not circles, so the nearest centre point can belong to the house opposite or the next road.
- Your location fix is rough. Indoors, or on a laptop without GPS, your position can be 50 m or more out. The tool warns you when your device reports low accuracy.
- You live in a large building or estate. A block of flats, a hospital, or a university campus can have several postcodes, one per entrance or department.
- The property is new. Brand-new homes have no postcode until the council has named the street and Royal Mail has allocated one.
The fix is the same each time: click your exact building on the map, or type your street address, and the tool looks up the postcode for that spot instead.
Check a postcode, including retired ones
Type a full UK postcode, with or without the space, and the tool shows where it is and which council, ward, and constituency it belongs to. If a form has rejected a postcode you are sure is right, it may have been retired: Royal Mail withdraws postcodes when streets are rebuilt or renumbered. For example, E1W 1UU in east London was retired in 2015. The tool recognises retired postcodes and tells you the year, so you know to look up the one in use now.
Postcodes elsewhere in Europe
- Germany, France, Spain, Italy: five digits, for example
10117for central Berlin or75007for the area around the Eiffel Tower. One code usually covers a whole district of a city or several villages, so it says much less about an exact address than a UK postcode does. - Netherlands: four digits and two letters (Dam Square in Amsterdam is
1012 NP), precise to a street or part of one, much like a UK postcode. - Ireland: Eircodes, introduced in 2015, are unique to every address. They are not in open data, so for Ireland the tool shows the address and links to the official Eircode finder.