What exactly does this validator check?

It checks three things. First, whether the postcode matches any of the six valid Royal Mail formats defined in BS 7666 (such as A9 9AA, A99 9AA, AA9 9AA, AA99 9AA, A9A 9AA and AA9A 9AA). Second, whether the two inward-unit letters avoid the six reserved characters (C, I, K, M, O and V) that Royal Mail excludes to prevent confusion with digits. Third, it looks up the Area code against the known Royal Mail postal area registry and flags unrecognised areas. It does not make a live Royal Mail PAF lookup, so it cannot confirm whether the postcode is currently active or has deliveries.

What is the difference between the outward code and the inward code?

Every UK postcode splits into two parts separated by a space. The outward code (e.g. SW1A) comes first and identifies the postal area and district that Royal Mail uses to route mail out from the sorting office. It contains the Area (letters only, e.g. SW) and the District (the digits and optional sub-district letter that follow, e.g. 1A). The inward code (e.g. 2AA) comes second and narrows delivery down to a specific street or building. Its first character is the Sector digit and the last two characters are the Unit.

Why are certain letters not allowed in the unit portion?

Royal Mail reserves C, I, K, M, O and V in the two-letter unit position to avoid handwriting or OCR ambiguity with digits. The letter O looks like zero, I looks like one, and so on. Any postcode with those letters in the unit position is structurally invalid under the specification, even if it looks superficially plausible.

What are GIR 0AA and BFPO postcodes?

GIR 0AA is the historic postcode for Girobank. It does not follow the standard pattern but is still routeable by Royal Mail and is explicitly listed in the specification as a valid exception. BFPO (British Forces Post Office) postcodes cover mail addressed to UK armed forces personnel overseas. They follow the format BFPO followed by one to four digits and are routed through the BFPO sorting centre in Northolt rather than through the standard geographic postcode network.

Does a valid result mean the postcode has real addresses?

No. A passing result means the postcode is structurally correct according to the Royal Mail format spec. Whether it corresponds to an active delivery point, a live property, or a currently assigned address is separate information held in the Royal Mail Postcode Address File (PAF). For address lookup and validation against real delivery points, use a PAF-based API. This tool checks format only.

Can I check Crown Dependencies such as Jersey, Guernsey and the Isle of Man?

Yes. JE (Jersey), GY (Guernsey) and IM (Isle of Man) are in the Royal Mail postal area table and this tool recognises them. Their postcodes follow the same BS 7666 format as mainland UK codes, so they pass the standard format check and are labelled with the correct Crown Dependency name.

UK Postcode Validator

Every address in England, Scotland, Wales, Northern Ireland and the Crown Dependencies carries a postcode that follows a strict format defined in Royal Mail BS 7666. Whether you are cleaning an address database, building a form with postcode validation, or just double-checking something from a handwritten envelope, this tool tells you instantly whether the format is correct and breaks the postcode down into its constituent parts — all in your browser, with nothing uploaded.

How UK postcodes are structured

A UK postcode always has two parts separated by a space: the outward code and the inward code.

The outward code has two sub-parts.

The Area is one or two letters at the very start (e.g. SW, EC, M, B). It identifies the broad postal area — M for Manchester, SW for South West London, AB for Aberdeen. There are around 120 postal areas in active use.
The District follows the area letters. It is one or two digits, sometimes with a trailing letter for a sub-district (e.g. 1A in SW1A or the plain 1 in SW1). The district tells Royal Mail which delivery office to route to within the area.

The inward code is always exactly three characters.

The Sector is a single digit (0–9) that subdivides the district for sorting.
The Unit is two letters that identify a specific street, part of a street, or even a single large building. Six letters are reserved and never appear in the unit position: C, I, K, M, O and V — they are excluded because they can be confused with digits in handwriting and OCR.

This gives six valid structural patterns:

Pattern	Example
A9 9AA	M1 1AA
A99 9AA	B15 1AB
AA9 9AA	SW1 1AA
AA99 9AA	SW1W 0NY
A9A 9AA	EC1A 1BB
AA9A 9AA	W1A 0AX

How the validator works

Step 1 — Normalisation. Spaces are stripped and the input is uppercased so that sw1a 2aa, SW1A2AA and SW1A 2AA all resolve identically.

Step 2 — Special postcode check. Two postcodes lie outside the standard pattern: GIR 0AA (the historic Girobank postcode) and BFPO codes (e.g. BFPO 801). These are checked first and returned as valid with a service label.

Step 3 — Regex match. The normalised string is tested against the full Royal Mail BS 7666 regex that encodes every valid combination of area letters, district digits, sector and unit.

Step 4 — Reserved-letter check. Even a structurally matching postcode fails if either inward-unit letter is from the reserved set (C, I, K, M, O, V).

Step 5 — Area lookup. The extracted Area code is looked up against the Royal Mail postal area table. Recognised areas get a town or region label; unrecognised areas are flagged (without failing the format check, since the format spec itself does not enumerate every area in its grammar).

Worked examples

SW1A 2AA — the postcode for 10 Downing Street.

Area: SW (South West London)
District: 1A (sub-district of SW1)
Sector: 2
Unit: AA (no reserved letters)
Result: Valid

EC1A 1BB — a City of London code using the A9A 9AA pattern.

Area: EC (East Central London)
District: 1A
Sector: 1
Unit: BB
Result: Valid

AA1 1CI — a plausible-looking code with a reserved unit letter.

Area: AA (not a real area — flagged as unknown)
Unit: CI contains I, which is reserved
Result: Invalid — reserved letter in unit

Postcode	Result	Reason
SW1A 2AA	Valid	Standard AA9A 9AA pattern, known area
M1 1AA	Valid	A9 9AA pattern, Manchester
GIR 0AA	Valid	Historic Girobank special case
BFPO 801	Valid	British Forces Post Office
EC1A 1BB	Valid	A9A 9AA sub-district pattern
ZE1 0AA	Valid	Shetland area
123 4AB	Invalid	Starts with digits, not letters
SW1A 2OA	Invalid	O is a reserved unit letter

Every check runs in your browser. No postcode you enter is ever sent anywhere.