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United Kingdom National Insurance number validator
How the National Insurance number format works
- Format
- 2 letters + 6 digits + 1 letter (A-D)
- Example
- AB123456C
Things to watch for
- Format-only check — there is no arithmetic checksum on a National Insurance number
- Certain prefix letter pairs (e.g. BG, GB, NK, KN, TN, NT, ZZ) are administratively reserved and never issued, even though they'd match a looser pattern
- D, F, I, Q, U and V never appear as the first letter, and O never appears as the second letter
- A valid format doesn't confirm the number is actually allocated to anyone
^(?!BG|GB|KN|NK|NT|TN|ZZ)[A-CEGHJ-PR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-D]$A UK National Insurance number identifies you to HMRC and DWP for tax, National Insurance contributions, and benefits — it’s the reference on your payslip, tax records, and any correspondence about state pension or benefit entitlement. It’s formatted as two letters, six digits, and a final letter from A to D, for example AB123456C, usually written with spaces (AB 12 34 56 C) for readability though the underlying value has none.
How this validator works
This tool checks the letter and digit pattern of a National Insurance number against the rules HMRC actually uses to allocate them: the two-letter prefix, six digits, and A–D suffix, plus the finer print most people don’t know exists. Certain letters — D, F, I, Q, U, and V — never appear as the first letter, O never appears as the second, and specific prefix pairs such as BG, GB, NK, KN, TN, NT, and ZZ are administratively reserved and never issued, even though they’d otherwise match the basic pattern. Everything runs client-side in your browser the moment you stop typing — nothing is sent anywhere or stored.
What a pass doesn’t tell you
A National Insurance number has no arithmetic checksum — no digit anywhere in it is calculated from the others, unlike a bank account number or an IBAN. So a pass here only means the number doesn’t violate any known format or prefix rule; it cannot confirm the number was actually issued or that it belongs to the person presenting it. Use this validator to catch an obviously mistyped or malformed NINO on a form before it causes a rejection — for anything beyond formatting, HMRC and DWP are the only authorities who can confirm a number is genuine.
Content last reviewed 2026-07-09.
National Insurance number FAQ
What does the final letter mean?
The last character of a National Insurance number is always a suffix letter from A to D. It doesn't encode any personal information — HMRC assigns it as part of the original allocation, and it's simply a fixed part of the format every valid number must end with.
Is there a checksum on a National Insurance number?
No. Unlike a bank sort code or an IBAN, a NINO has no arithmetic check digit. Its validity rests entirely on format rules: which letters can open it, which letter pairs are reserved and never issued, and which final letter it ends with.
Does a valid format mean it's a real, issued NINO?
No. A correctly formatted number only means it doesn't break any of the known prefix or structure rules — it doesn't confirm the number has actually been allocated or that it belongs to the person who gave it to you. Only HMRC or DWP can confirm that.