NI Number Checker
Check UK National Insurance number format and prefix rules.
About this tool
Paste NI numbers, one per line, with or without spaces. Each is checked against HMRC's format rules, formatted as AB 12 34 56 C if valid, or given a plain-English reason if not.
The rules behind NI numbers
A UK National Insurance number (NINO) is used for tax, National Insurance contributions and benefits. Its format is two prefix letters, six digits and a suffix letter, as in AB 12 34 56 C. There's no check digit, but HMRC excludes many combinations. The first letter can't be D, F, I, Q, U or V. The second can't be D, F, I, O, Q, U or V. The prefixes BG, GB, KN, NK, NT and ZZ are never issued, and TN indicates a temporary reference rather than a real number. The suffix is always A, B, C or D. HMRC's own documentation uses QQ 12 34 56 C as an example because QQ is never issued, so the checker correctly rejects it. Validating these rules on payroll, HR and onboarding forms catches typos and placeholder numbers before they cause RTI submission errors. Honest limits: passing the format check doesn't mean a number has been issued or belongs to the person who gave it. Employers should confirm details through official channels, and only HMRC and DWP hold the register. NI numbers are personal data, so this runs entirely in your browser. Not tax advice. The NHS number validator, the UK postcode checker and the paycheck calculator are related.
Frequently asked questions
What is the format of a National Insurance number?
Two letters, six digits and a letter A–D, e.g. AB 12 34 56 C.
Which NI number prefixes are invalid?
First letter D, F, I, Q, U or V; second letter D, F, I, O, Q, U or V; and BG, GB, KN, NK, NT, TN and ZZ.
Can I check whom an NI number belongs to?
No. Only HMRC and DWP can confirm that. This checks the format only.
Is my data sent anywhere?
No. Everything is checked in your browser, and nothing is stored, looked up online or sent.