There is a string of numbers and letters near the top of your payslip that most people have never once looked at, and getting it wrong can cost you several hundred pounds a year without a single alarm going off. It is your tax code, and HMRC issues it to your employer to decide how much income tax comes out of every pay packet. When it is wrong — and for a surprising number of workers it is — you either overpay quietly all year or get hit with an unexpected bill later.
HMRC processes tax for more than 30 million people through PAYE, and codes get out of date for ordinary reasons: you changed jobs, you started a second job, a workplace benefit ended, or the figures simply did not update when your circumstances did. Nobody emails you to say "by the way, you have been overpaying since April." You have to check it yourself. The good news is that it takes about ten minutes, and if you are owed money, claiming it back is genuinely straightforward.
What the code actually means
The standard tax code for the 2026/27 tax year is 1257L. That reflects the personal allowance — the £12,570 you can earn before paying any income tax — which has been frozen at that level for several years now and is staying frozen, a freeze that quietly drags more people into higher tax bands as wages rise. The number is your allowance with the last digit knocked off; the letter tells HMRC about your situation.
A quick translation of the letters you are most likely to see:
- L — you get the standard personal allowance. This is the common one.
- BR — every pound is taxed at the basic 20% rate with no allowance. Correct for a second job, but wrong (and expensive) if it lands on your main job.
- D0 / D1 — taxed at the higher (40%) or additional (45%) rate with no allowance, again usually for a second income.
- 0T — no personal allowance at all, often applied when a new employer has no details yet. If this is on your only job, you are very likely overpaying right now.
- K — you have income or benefits that exceed your allowance (a company car, say), so the code adds to your taxable pay rather than subtracting from it.
- W1 / M1 / X — an emergency code applied on a "week one / month one" basis. Common after a job change and meant to be temporary.
The single most important check: if you have one job and your code is anything other than 1257L (or close to it for a clean reason), ask why. An emergency code or a 0T sitting on your only income is the classic way people end up hundreds of pounds down without realising.
Where to find it and how to check
You will find your code on your payslip, on your P60 at the end of the tax year, on a P45 when you leave a job, and in any "coding notice" letter HMRC sends. The most reliable place, though, is your Personal Tax Account on the GOV.UK website or the official HMRC app. Sign in with your Government Gateway details and you can see your current code, the income HMRC thinks you have, and the figures it used to build the code.
This is where mistakes jump out. A common one: HMRC is still counting a company benefit you no longer get, or it has split your allowance across an old job you left months ago. Another is two jobs where the allowance has been applied to the wrong one — it should usually sit against your higher-earning job, not the part-time weekend role. If the numbers feeding your code do not match your real life, that is your signal.
Claiming back what you have overpaid
If you have paid too much, you are not chasing a favour — it is your money. How you get it back depends on the year.
For the current tax year, the usual route is to tell HMRC the code is wrong, either through your Personal Tax Account, the app, or by phone. HMRC corrects the code, sends a new one to your employer, and the overpayment comes back to you automatically through your wages over the remaining months — you simply pay less tax until it balances out.
For previous tax years, the process is different and worth knowing about because you can go back up to four years. After a tax year ends, HMRC often reconciles your records and, if you overpaid, sends a P800 letter telling you a refund is due. If you get one, you can usually claim the money online and have it in your bank within about five working days — do not wait for a cheque if the online option is offered. If you think you overpaid in an earlier year and never got a P800, you can write to or call HMRC and ask them to check.
One firm warning that is more relevant than ever: ignore the "tax refund" texts and emails. HMRC does not notify you of refunds by text, by email with a link, or through social media, and the genuine reclaim is always free. Every "claim your rebate" advert promising to do it for you is taking a cut — often 30% to 48% plus fees — of a refund you could get yourself in fifteen minutes for nothing. There is no reason to hand a third party hundreds of pounds to fill in a form on GOV.UK that you are perfectly capable of completing.
While you are in there, claim the things people forget
Checking your code is also the moment to grab two reliefs that quietly go unclaimed. The first is Marriage Allowance: if one partner earns under the personal allowance and the other is a basic-rate taxpayer, the lower earner can transfer £1,260 of allowance, worth up to £252 a year, and you can backdate it four years for a lump sum running into four figures.
The second is work expenses. If you pay for things wholly for your job that your employer does not reimburse — professional subscriptions, a uniform you wash yourself, mileage in your own car for work travel — you may be due relief through your code. It is not glamorous, and any single one is small. But a wrong code plus a missed allowance plus unclaimed expenses, compounded over a few frozen-allowance years, is exactly the kind of slow leak that a ten-minute look at your Personal Tax Account plugs for good.
The traps that catch second jobs and the self-employed side hustle
Second incomes are where codes go wrong most often, and the post-pandemic rise in side hustles has made it worse. The system is built on the assumption that your personal allowance sits against one job. If you pick up a second PAYE role — bar work, weekend retail, a bit of tutoring run through an agency — that second employer should be putting you on a BR code so the allowance is not accidentally claimed twice. The error people hit is the reverse: the allowance ends up split or duplicated, and a year later HMRC reconciles the records and sends a bill for tax that was never deducted. Getting the allowance assigned to the higher-paying job from the start avoids the nasty surprise.
If your side income is self-employed rather than PAYE, the wrinkle is different. Once you earn more than £1,000 from self-employment in a tax year — the trading allowance — you generally need to register for Self Assessment and report it, and HMRC can collect any tax due by adjusting your PAYE code the following year rather than asking for a separate payment. That is convenient, but it also means a quiet code change can appear that you did not expect. If your take-home pay drops one month for no obvious reason, your code is the first thing to check, not the last.
It is also worth knowing that the High Income Child Benefit Charge and the tapering of the personal allowance above £100,000 both get clawed back through your code or through Self Assessment. If you are anywhere near those thresholds, a code that has not kept pace with a pay rise can leave you with a bill that lands all at once. None of this is a reason to panic. It is a reason to treat the code on your payslip as something you actually read, once or twice a year, rather than a string of characters you have trained yourself to scroll past.