Every UK postcode is made of two halves separated by a space — the outward code and the inward code. Each half narrows the location further, forming a four-level hierarchy.
The four levels
- Area — one or two letters (e.g. SW), the broadest grouping
- District — area plus a number (e.g. SW1), a postal district
- Sector — district plus the first inward digit (e.g. SW1A 1)
- Unit — the final two letters (e.g. ...AA), a small group of addresses
Why the hierarchy matters
This structure lets Royal Mail sort efficiently and lets software resolve a postcode to a precise address list. It's also why a single postcode can be turned into a full list of properties — see the postcode to address API.
Go deeper
For the exact character rules and validation patterns, read UK postcode format explained, or return to the PAF guide hub.
Frequently asked questions
How does the UK postcode system work?
Each postcode has an outward code (area + district) and an inward code (sector + unit). Together they narrow a location from a wide postal area down to a small cluster of addresses.
What is the outward and inward code?
The outward code (e.g. SW1A) identifies the postal district handled by a sorting office; the inward code (e.g. 1AA) identifies the delivery point group within that district.
What are postcode areas, districts and sectors?
Areas are the broadest level (e.g. SW), districts add a number (SW1), sectors add the first inward digit (SW1A 1), and units are the final two letters identifying the smallest group of addresses.
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