Check a National Insurance number’s format in seconds
This free National Insurance number check tells you whether a UK NI number is written in a valid format. It applies HMRC’s published rules: two prefix letters (some letters and prefixes are never used), six digits and a final letter A, B, C or D. Type a number, or paste a whole list from a payroll export and download the results as CSV.
It is useful for catching typing mistakes before you submit a payroll file, a pension enrolment or an application form. It checks the format only — it cannot tell you whether HMRC has issued a number, or who it belongs to — and everything runs in your browser, so the numbers you check are never sent anywhere.
How to use the UK National Insurance Number Format Checker
- Type the National Insurance number; spaces, hyphens and lower case are fine.
- Read the verdict and the list of checks: prefix, letters, digits and suffix.
- Copy the number in the standard “AB 12 34 56 C” spacing if you need it.
- For many numbers, switch to “A list”, paste one per line and press “Check the list”.
- Download the CSV to fix the invalid rows in your spreadsheet.
UK National Insurance Number Format Checker features
HMRC’s prefix rules
D, F, I, Q, U and V are never used as either prefix letter, O never as the second, and BG, GB, KN, NK, NT, TN and ZZ are not allocated.
Clear reasons
Each failed rule is explained, such as “The suffix letter is missing” or “Prefix GB is not used”.
Administrative references flagged
NC, PY and PZ numbers were used as dummy or administrative references and are noted as such.
Bulk check with CSV
Up to 5,000 lines, with a formula-injection guard on every cell of the export.
Private by design
No uploads, no storage — the numbers stay in this tab.
National Insurance number format (HMRC NIM39110)
HMRC’s own example, QQ 12 34 56 C, is deliberately invalid (Q is never used) so it can be printed safely. Temporary “TN” numbers are no longer used: employers may not use them, and they are not accepted under the Quality Standard. Rules from the HMRC National Insurance Manual, NIM39110, updated 7 October 2026.
| Part | Rule | Example |
|---|---|---|
| Prefix | Two letters. D, F, I, Q, U, V are never used as either letter; O is never the second letter; BG, GB, KN, NK, NT, TN, ZZ are not used | AB, JG, OA |
| Number | Six digits, usually written in pairs | 12 34 56 |
| Suffix | Always A, B, C or D | C |
What this check can and cannot tell you
- Can: spot a mistyped letter, a missing suffix, a digit typed as a letter, or a prefix that is never issued.
- Cannot: confirm the number was issued, that it is still active or whose it is — there is no check digit in a National Insurance number.
- Employers verify a new starter’s number through HMRC’s National Insurance Number Verification Request (via payroll software) and right-to-work checks, not with a format check.
- If you have lost your own number, find it on GOV.UK, in the HMRC app, or on a payslip, P60 or letter from HMRC or DWP.
UK National Insurance Number Format Checker FAQ
What is the format of a National Insurance number?
Two letters, six digits and a final letter A, B, C or D — for example AB 12 34 56 C. This National Insurance number checker applies HMRC’s rules for which prefix letters are allowed.
Is this national insurance number check free?
Yes. It is free, needs no sign-up and runs in your browser; nothing you type is sent or stored.
Can it tell me if a National Insurance number is real?
No. It checks the format only. HMRC confirms numbers for employers through payroll software; individuals can look up their own number on GOV.UK.
Which prefixes are invalid?
Prefixes with D, F, I, Q, U or V as either letter, O as the second letter, and BG, GB, KN, NK, NT, TN and ZZ.
Why does my number have no letter at the end?
Some forms drop the suffix, but every NI number has one (A, B, C or D). Check a payslip or P60 for the full number.
Can I check a list of numbers?
Yes — switch to “A list”, paste one number per line, and download the results as CSV.