The UK Postcode Format

UK postcodes are made up of five to seven alphanumeric characters, created by Royal Mail. A full postcode identifies an area with many addresses or a single delivery point. Understanding the format helps you validate postcodes, build forms, and use tools like our area insights, crime data, and price paid data correctly.

Key patterns

Every UK postcode fits one of these patterns. The first part is the outward code (before the space), the second is the inward code (exactly three characters after the space).

Outward code

The outward code is the part of the postcode before the space. It identifies the town or area. Some outward codes are non-geographic (e.g. for PO boxes).

• Always starts with a letter
• May end with a number or a letter
• Between 2 and 4 characters long

Examples: SE1P, RH10, W1A, L1.

Inward code

The inward code is the part after the space. It helps sort and deliver mail within the outward area.

• Always starts with a digit
• Exactly 3 characters long (one digit + two letters)

Examples: 8JQ, 7HF, 0NY.

Postcode area

The postcode area is the longest initial string of letters in the postcode. It usually corresponds to a town, city or region.

Examples: BT (Northern Ireland), EH (Edinburgh), L (Liverpool), SW (South West London).

District code

The district code is part of the outward code: the area letters plus the numbers, but not the trailing letter (if any). It is 2–4 characters.

Examples: SE1, RH10, W1, L1.

Sub-district code

The sub-district code appears only in some outward codes, mainly in dense London areas. It includes the trailing letter omitted from the district (3–4 characters).

Examples: SE1P, E1W, NW1W, EC1A, W1A.

Postcode sector

The postcode sector is the outward code plus a space plus the first character of the inward code (the digit). It is 4–6 characters including the space.

Examples: CV1 4, L1 8, SW1W 0.

Postcode unit

The postcode unit is the last two letters of the postcode. It usually represents a small set of addresses, a single property, or part of a street.

Examples: JQ (from L1 8JQ), HF (from GU16 7HF), NY (from SW1W 0NY).

How to use UK postcodes

Valid UK postcodes have 5, 6, or 7 characters (ignoring the space). You can normalise input by removing spaces and hyphens, then inserting a space before the last three characters (the inward code). Use this when building search forms or calling APIs—for example on our map search, nearest stations, and postcode report tools.

This guide is based on Royal Mail’s postcode system and common usage. For official validation and address lookup, use Royal Mail’s or other authorised services. Our data sources page explains where we get our postcode and property data.