How to Use the UK NI Number Format Validator

Type or paste a National Insurance number into the box above — with or without spaces, either works.
Click Check. The tool verifies the two-letter prefix, six digits, and suffix letter against HMRC's documented rules.
Read the decode table for the prefix letters, digit block, and suffix letter.
Nothing you type is uploaded anywhere — the whole check runs as plain JavaScript in your browser tab.

The National Insurance Number Format

A UK National Insurance (NI) number follows the pattern AA 12 34 56 C — two letters, six digits, one letter — and is used to track an individual's National Insurance contributions and tax record throughout their working life. The format has real, documented constraints beyond "any two letters":

Prefix letters: The first two letters can each be any of A, B, C, E, G, H, J, K, L, M, N, O, P, R, S, T, W, X, Y, or Z. The letters D, F, I, Q, U, and V are never used as either prefix letter, and the second letter additionally excludes O.
Reserved/banned prefix pairs: HMRC never issues NI numbers starting with BG, GB, NK, KN, TN, NT, or ZZ — several of these are reserved for administrative use (temporary/placeholder numbers), and ZZ is used in official examples and training materials specifically because it's guaranteed never to be a real number.
Digits: Exactly six numeric digits, no additional structure.
Suffix letter: Must be A, B, C, or D — historically used to indicate which quarter of the tax year a person's NI record began, though this distinction has little practical relevance today. Temporary numbers sometimes omit the suffix entirely.

Worked Example

AB123456C

matches the full pattern: prefix AB (both letters valid, and "AB" isn't a banned pair), six digits 123456, and a valid suffix letter C.

Why There's No Checksum

Unlike GSTIN or Aadhaar, NI numbers are issued sequentially by HMRC rather than generated with a self-checking mathematical formula — there is no published check-digit algorithm that lets you verify a specific six-digit block is "correct" the way Verhoeff does for Aadhaar. That means this tool (and any tool claiming to validate NI numbers) can only catch format-level mistakes — a banned prefix, an invalid letter, a wrong suffix — not a single mistyped digit in the middle of an otherwise well-formed number.

What This Tool Does NOT Do

This checks format only. It does not query HMRC's systems, does not confirm a number is actually issued, and cannot tell you whether it belongs to a specific person. A number that passes this check is merely well-formed — for anything that matters (employment, benefits, tax), always verify through official channels (HMRC or gov.uk).

Common Mistakes

Using D, F, I, Q, U, or V as a prefix letter — none of these ever appear in a real NI number
Not realizing "O" is banned specifically as the second prefix letter even though it's fine as the first
Typing a banned pair like "GB" out of habit (since GB is the UK's own country code) — HMRC deliberately never issues it as an NI prefix
Assuming a passing format check means the number is real — there's no checksum to confirm that

Frequently Asked Questions

Is my NI number uploaded anywhere? No — everything runs locally in your browser tab.

Why doesn't this tool have a checksum check like the Aadhaar or GSTIN tools? Because none exists — HMRC assigns NI numbers sequentially, not via a self-verifying formula.

Does a valid format mean the number is real? No — it means the shape is correct, not that HMRC has actually issued it.

Is this tool free? Yes, completely free, with no login and no usage limit.