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Password Strength Checker
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UK National Insurance Number Checker
Checks a UK National Insurance number against the prefixes that are never issued — it has no check digit.
Security
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In short
A UK National Insurance number has no check digit, so its validity cannot be established offline. What can be checked is the prefix, and the rules are specific: the letters D, F, I, Q, U and V never appear in the first position, the same set plus O never appears in the second, and the combinations BG, GB, NK, KN, TN, NT and ZZ are never issued at all. The final letter is A, B, C or D and is optional in many systems — it carries no information and was historically a contribution-cycle marker. Anything passing those rules is possible; only HMRC can confirm it was issued.
How to use the UK National Insurance Number Checker
- Paste the number — punctuation and spaces are ignored.
- Read the verdict and, where the number encodes anything, what it decodes to.
- Treat a pass as "typed correctly", not as "confirmed by the authority".
About the UK National Insurance Number Checker
A UK National Insurance number has no check digit, so its validity cannot be established offline. What can be checked is the prefix, and the rules are specific: the letters D, F, I, Q, U and V never appear in the first position, the same set plus O never appears in the second, and the combinations BG, GB, NK, KN, TN, NT and ZZ are never issued at all. The final letter is A, B, C or D and is optional in many systems — it carries no information and was historically a contribution-cycle marker. Anything passing those rules is possible; only HMRC can confirm it was issued.
What people use it for
- Catching a mistyped number in a form before it reaches a database.
- Cleaning an imported list and finding the rows that cannot be right.
- Understanding what the number encodes and what it does not.
Details
| Shape | Two letters, six digits, and usually a final letter A to D |
|---|---|
| Check digit | None |
| Never first or second | D, F, I, Q, U, V — and O is also excluded from the second position |
| Never issued | BG, GB, NK, KN, TN, NT and ZZ |
| Final letter | Optional, carries no information |
The prefix rules are strong enough to reject a meaningful share of invented numbers, which is why they are worth applying in a form even though they cannot confirm anything. Nothing you paste is transmitted — the check runs entirely in your browser.
Where this tool stops
Format and prefix rules only. A NINO has no check digit and cannot be validated offline.
Frequently asked questions
Is the last letter part of my number?
Yes, but it carries no information — it was a contribution-cycle marker and many systems accept the number without it. A NINO written with or without the suffix refers to the same person.
Why are some prefixes never used?
Mostly to avoid ambiguity: D, F, I, Q, U and V are excluded because they are easily confused with other characters or were reserved. BG, GB, NK, KN, TN, NT and ZZ are excluded by administrative decision, ZZ because it is used as a placeholder.
Can I check if a NINO is real?
Not offline. There is no check digit. The prefix rules can prove a number was never issued, which is useful in a form, but cannot prove one was. HMRC is the only authority.
Is anything I paste sent anywhere?
No. The arithmetic runs in this browser tab and no request is made — open your Network tab and watch while you type. That matters more for an identity number than for almost anything else on this site, which is why there is no server side to this tool at all.
Does this replace an official check?
No, and it cannot. Every scheme here proves at most that the digits are internally consistent. Only the issuing authority can confirm a number exists, is active, and belongs to a particular person.
Is the UK National Insurance Number Checker free?
Free, unlimited, and no email address required. You can use it a hundred times a day and nothing changes.
Does anything I enter leave my computer?
No. The UK National Insurance Number Checker runs entirely in your browser using JavaScript. Because the work happens locally, the page also keeps working if your connection drops, and closing the tab is all it takes to discard everything.
When would I actually need the UK National Insurance Number Checker?
The jobs it is built for: catching a mistyped number in a form before it reaches a database. Cleaning an imported list and finding the rows that cannot be right. Understanding what the number encodes and what it does not. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Is there anything the UK National Insurance Number Checker cannot do?
Format and prefix rules only. A NINO has no check digit and cannot be validated offline. Knowing where a tool stops being trustworthy matters more than the cases where it works.
What figures does the UK National Insurance Number Checker work from?
Shape is Two letters, six digits, and usually a final letter A to D. Check digit is None. Never first or second is D, F, I, Q, U, V — and O is also excluded from the second position. These are printed on the page itself, so you do not have to take them on trust.
Will this work on a tablet?
Yes. The page is responsive and the page works in Chrome, Safari, Firefox, Edge and Samsung Internet on both Android and iOS. There is no app to install and nothing to sign into.
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