UK Postcode Checker: Format & Code Breakdown
Normalise a UK postcode and split its outward code, area, district, sector and unit. This checks structure only—it does not invent a post town or claim that a postcode or address exists.
Private by design: the check runs in your browser without a postcode API.
Format and structure
Check a UK postcode
How to read the result
Normalise the entry
Case, spaces, full stops, brackets and common dashes are normalised into the standard outward-space-inward display.
Check the structure
The code is compared with the published unit-postcode shape, including the permitted inward-code letters.
Verify the record
Use Royal Mail for an address and current postal status, or an ONS directory release for postcode-to-geography analysis.
UK postcode structure
ONS describes a unit postcode as an alphanumeric reference with a two-to-four-character outward code and a three-character inward code. A single space separates the two in the normal display. The hierarchy is useful, but it should not be confused with an administrative boundary or a town database.
Area
PO
Leading outward letters
District
PO15
Complete outward code
Sector
PO15 5
Outward + inward digit
Unit
PO15 5RR
Complete postcode
Postal areas and districts support mail routing. They are not guaranteed to line up with counties, councils, wards or settlements. ONS assigns directory postcodes to statistical and administrative geographies using documented methods, and warns that postcode boundaries change continuously as addresses and delivery needs change.
The checker therefore reports codes, not a guessed location. The previous version attached one post town and region to every code sharing the same first letters. That was too broad: an area can contain many post towns, while ONS explicitly says it does not maintain a postcode-to-town lookup because there is no fixed definition of towns or urban areas.
Six conventional unit-postcode shapes
| Shape | Example |
|---|---|
| A9 9AA | M1 1AE |
| A99 9AA | M60 1NW |
| A9A 9AA | W1A 1HQ |
| AA9 9AA | CR2 6XH |
| AA99 9AA | DN55 1PT |
| AA9A 9AA | EC1A 1BB |
Here A means a letter and 9 means a digit. This shape table is a syntax reference, not a list of assigned postcodes. The special GIR 0AA code is handled separately.
Primary sources and official lookup
- Royal Mail — Postcode FinderOfficial address and postcode search backed by Royal Mail address information.
- Office for National Statistics — postal geographiesOutward/inward hierarchy, counts, geography use and boundary limitations.
- Office for National Statistics — postcode productsONSPD and NSPL scope, update cadence, methods, formats and licensing links.
- GOV.UK Design System — addressesUser-centred input normalisation and address-lookup guidance.
Structure and guidance checked 26 August 2026. Royal Mail and ONS data can change; use their current products for operational decisions.
Frequently asked questions
Does a green result mean the postcode exists?
No. It means the characters fit a recognised UK unit-postcode structure. This browser tool does not contain Royal Mail’s Postcode Address File or a current ONS Postcode Directory release, so it cannot prove that the code is assigned, live, terminated or connected to a particular address.
Can the postcode area identify the post town?
Not reliably. The leading area letters are part of the postal hierarchy, but an area can cover many towns and may cross administrative boundaries. ONS says it does not maintain a separate postcode-to-town lookup because there is no fixed definition of towns or urban areas. Use Royal Mail Postcode Finder for postal-address information.
What is the outward code?
The outward code is the two-to-four-character portion before the space, such as PO15 or EC1A. It contains the postcode area and district. It routes mail towards the correct sorting area and district, but it is not a complete unit postcode.
What is the inward code?
The inward code is the final digit and two letters, such as 5RR. Its first digit combines with the outward code to form the postcode sector. The final two letters form the unit portion. The letters C, I, K, M, O and V are not used in the inward-code letter positions.
Why are spaces and punctuation accepted?
GOV.UK’s address-lookup guidance recommends accepting upper or lower case, missing or extra spaces and common punctuation instead of rejecting a recognisable entry. This checker removes spaces, full stops, brackets and common dashes, then displays one space before the inward code.
Does the checker cover BFPO numbers?
No. British Forces Post Office routing numbers use a separate BFPO format and are not ordinary geographic unit postcodes. This page checks conventional UK unit-postcode structure plus the special GIR 0AA code.
Can I use a postcode as an exact map coordinate?
No. Royal Mail notes that postcodes are not intended to pinpoint an exact geographical location. ONS directory coordinates and administrative assignments represent postcode-level reference points and have documented boundary and data-currency limitations.
Is the postcode sent anywhere?
No. Normalisation and parsing run locally in your browser. This tool does not call a postcode API or upload your entry. If you follow the Royal Mail or ONS links, their own privacy and usage terms apply.
Important limitation
This is a local syntax checker and structural decoder, not Royal Mail PAF, ONSPD, NSPL, an address validator, a delivery guarantee or proof of residence. It does not identify a post town, street, property, council, coordinates or current postcode status. Kalcify is not affiliated with or endorsed by Royal Mail or ONS.
Published by Kalcify · Last updated
Calculators are built against primary sources — government tax authorities (IRS, HMRC, CRA, ATO) for finance and the World Health Organization for health metrics. Updated when rates or rules change. View methodology and data sources.
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