Take SW1A 1AA. The space splits it into the outward code (SW1A) and the inward code (1AA). Within those, four levels of detail emerge.
The four parts
- Area (SW) — one or two letters, the widest grouping (London South West)
- District (SW1A) — adds digits/letters to identify the postal district
- Sector (SW1A 1) — the first inward digit narrows it further
- Unit (AA) — the final two letters: a handful of addresses
Why the structure exists
This hierarchy lets Royal Mail sort efficiently and lets software resolve a postcode to a precise address list. For the formal validation rules, see UK postcode format explained.
Explore further
Browse the full UK postcode areas list or return to the PAF guide hub.
Frequently asked questions
What are the parts of a UK postcode?
A UK postcode has an outward code (area + district) and an inward code (sector + unit). For SW1A 1AA: SW is the area, SW1A the district, SW1A 1 the sector, and AA the unit.
What is the outward code?
The outward code is the first half of a postcode (e.g. SW1A). It identifies the postal district routed to a particular sorting office.
What is the inward code?
The inward code is the second half (e.g. 1AA): a digit identifying the sector plus two letters identifying the unit — a small group of addresses.
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