The four parts of a postcode: PE29 3BU taken apart
What is a postcode district and a postcode sector?
A postcode district is the half of a postcode before the space: PE29 in PE29 3BU. It is also called the outward code. A postcode sector is the district plus the first digit after the space: PE29 3.
The half after the space, 3BU, is the inward code, and it is always three characters.
Valid UK postcode formats, with an example of each
The inward code is always a digit followed by two letters. The outward code varies, which gives six shapes. A stands for a letter and 9 for a digit.
| Shape | Outward code | Example from our data |
|---|---|---|
| A9 9AA | One letter, one digit | B1 1AY |
| A99 9AA | One letter, two digits | B10 0AB |
| AA9 9AA | Two letters, one digit | AL1 1AG |
| AA99 9AA | Two letters, two digits | AB10 1AB |
| A9A 9AA | One letter, one digit, one letter | E1W 1AA |
| AA9A 9AA | Two letters, one digit, one letter | EC1A 1AA |
In the data we hold, the shapes with a letter on the end of the outward code are used in these postcode areas: E, EC, N, NW, SE, SW, W and WC.
Common mistakes
- The letter O for a zero. The inward code always starts with a digit, so the character after the space is never a letter.
- The missing space. The space goes before the last three characters. PE293BU and PE29 3BU are the same postcode, and this site accepts both.
- Small letters. A postcode is written in capitals, but pe29 3bu is read as the same postcode here.
What a postcode is not
A postcode is a group of addresses for delivering post. It is not a boundary. It can cross a ward or a council line, and it does not say which street an address is on.
That is why this site gives the one council and the one ward that ONS places a postcode in, and says that is what it is doing.
How many postcodes are there in the UK?Where this comes from
- The parts of a postcode Postal geography, at the Office for National Statistics
- The examples and the shapes in use ONS Postcode Directory, August 2026
Published 7 October 2026 by Towpath Digital Ltd, the company that runs CheckAPostcode. Last checked against these sources on 8 October 2026. If something here is out of date, report an error.