Understanding Your National Insurance Number
What it is, how you get one, and what to do if you've lost it, a sister page to Understanding Your Tax Code, covering the number rather than the deductions
Your National Insurance number, something like QQ 12 34 56 C, has no direct influence on the rates and thresholds covered on National Insurance Explained, but is rather a personal identifier that stays with you for life. This page covers the number itself: what it's for, how you get one, and what to do if you've lost it.
What it is (and isn't)
Your National Insurance number exists for one reason: to make sure your National Insurance contributions and tax are recorded against your name, and yours alone. HMRC, DWP, your employer, and your pension provider all use it to keep your record straight as you move between jobs over your working life.
It is not proof of your identity, and it's not proof that you have the right to work in the UK, a common misconception. Employers checking right to work need to see specific identity or immigration documents; a National Insurance number on its own doesn't satisfy that check.
The format, decoded
A National Insurance number always has the same shape: two letters, six digits, then one final letter.
Prefix
No personal meaning. Some letters (D, F, I, Q, U, V) are never used, and combinations like GB, BG, or NK never appear.
Six digits
Issued in sequence, not your birth date or region, a common myth.
Suffix
Always A, B, C, or D. Left over from when paper contribution cards were returned annually, staggered across the year.
The example above, QQ 12 34 56 C, is gov.uk's own placeholder. QQ is a prefix that's never actually issued to anyone, chosen deliberately so official examples can never risk showing a real person's number.
When and how you get one
If you live in the UK, you're normally sent a National Insurance number automatically in the three months before your 16th birthday, as long as a parent or guardian has claimed Child Benefit for you. The letter goes to whatever address HMRC holds at the time.
If you weren't sent one automatically, most commonly because you've moved to the UK from abroad, you can apply online once you're living in the UK, have the right to work, and are working, looking for work, or have a confirmed job offer. Applications typically take up to four weeks. You don't have to wait for the number to arrive before starting a job: you can begin work as soon as you can prove your right to work another way, and give HMRC your National Insurance number once it arrives.
Temporary numbers on your payslip
If you start a job before your real National Insurance number has come through, your employer's payroll software sometimes needs a placeholder to process your pay in the meantime. HMRC reserves certain prefixes, including TN, specifically so they're never issued as real numbers, which is what makes them safe to use as a temporary stand-in without ever colliding with someone's genuine number. If you see one of these on an early payslip, it's expected. Just make sure your employer updates it to your real number once it arrives, since HMRC needs the correct one to keep your record straight.
If you've lost it
You almost certainly don't need to do anything drastic, your number doesn't change, so it's just a case of finding it again. Check a recent payslip, your P60, or any letter from HMRC about tax, pensions, or benefits; it's also viewable in the HMRC app and your personal tax account if you've set one up. If none of those work, HMRC can post you a letter confirming your number.
Two things to know before you go looking: HMRC will not tell you your number over the phone or via webchat, for security reasons. It's accessible via a letter or digital account only. Physical National Insurance cards were phased out years ago, so there's no card to have reissued even if you remember once having one; a letter is the only replacement now.
Keeping it safe
Because it's a permanent, unique identifier, your National Insurance number is a useful piece of the puzzle for identity fraud when combined with other personal details: enough, in the wrong hands, to help someone commit benefit or tax fraud in your name. Only share it with organisations that have a genuine need for it, such as your employer, HMRC, DWP, your pension provider, and some financial services firms. Be wary of anyone else asking for it unprompted, especially by phone, text, or email.
This page explains general National Insurance number rules and is not personalised advice; see the disclaimer.