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NIN

General

UK National Insurance Number

A unique personal reference number issued by HMRC used to track National Insurance contributions and tax records in the UK. Format: 2 letters, 6 digits, 1 letter.

Definition

A National Insurance Number (NIN or NINO) is a unique personal reference number issued by HMRC, used to track an individual's National Insurance contributions, tax records, and eligibility for UK state benefits. It's required for nearly every formal employment and tax interaction in the UK, from starting a new job to claiming certain government benefits.

The NIN format follows specific, documented rules beyond just "2 letters, 6 digits, 1 letter" — several letter combinations are explicitly excluded by HMRC's numbering scheme, which makes it possible to catch many typos or invalid numbers through format checking alone, without needing to contact HMRC. The UK NIN Validator applies these exact rules.

Formula

A NIN is checked against four structural rules:

  1. Format: 2 letters, 6 digits, 1 letter.
  2. First letter: must not be D, F, I, Q, U, or V.
  3. Second letter: must not be D, F, I, Q, U, V, or O.
  4. Prefix: the first two letters together must not be BG, GB, NK, KN, TN, NT, or ZZ.
  5. Suffix: the final letter must be A, B, C, or D.

Worked Example

For the NIN AB 12 34 56 C, normalised to AB123456C:

  • First letter "A" — not in the banned list ✓
  • Second letter "B" — not in the banned list ✓
  • Prefix "AB" — not a reserved combination ✓
  • Suffix "C" — one of the valid suffix letters ✓

All checks pass, so the NIN is structurally valid.

A NIN starting with GB, by contrast, fails immediately — GB is one of the seven permanently reserved prefixes.

Key Things to Know

  • Letter exclusions are specific, not arbitrary: D, F, I, Q, U, V (first letter) and additionally O (second letter) are never used, by design.
  • Seven prefixes are permanently reserved: BG, GB, NK, KN, TN, NT, and ZZ are never allocated to a real NIN.
  • Suffix must be A–D: any other final letter makes the NIN invalid.
  • A valid format isn't proof of an active NIN: confirming a NIN is actually issued and active requires verification through HMRC, not just a format check.

Related Terms

SINCanada Social Insurance NumberPANPermanent Account Number

Frequently Asked Questions

A NIN follows the pattern 2 letters, 6 digits, 1 letter — for example, AB123456C. It's commonly printed with spaces in pairs, such as AB 12 34 56 C.
HMRC excludes the letters D, F, I, Q, U, and V from the first letter position, and additionally excludes O from the second letter position, as part of the official numbering scheme. These exclusions help avoid confusion with similar-looking characters.
BG, GB, NK, KN, TN, NT, and ZZ are permanently reserved two-letter prefixes that are never allocated to a real NIN. A number starting with any of these is automatically invalid.
The final letter historically indicated how often National Insurance contributions were collected, though it no longer carries that operational meaning today. It must still be one of A, B, C, or D for the number to be considered valid.
Your NIN appears on your payslip, P60, tax return, or any official HMRC correspondence. If you've lost it, you can request confirmation through HMRC's official online services.