Tax Code Explained — What 1257L and Every Other Code Means
Your tax code is the single line on your payslip that decides how much tax you pay — and the one almost nobody can read. This tax code explained guide decodes the numbers, every letter, the emergency and K codes, and why yours changed.
How to read a tax code in ten seconds
A tax code has two parts. The number, multiplied by ten, is the amount you can earn tax-free this year: code 1257 means £12,570 — the standard personal allowance. The letter explains how you got that allowance: L is the ordinary allowance with nothing unusual attached. So 1257L, the code most UK employees including most NHS staff are on, simply says “this person gets the full standard personal allowance of £12,570, spread evenly across the year”. Everything else — every other letter, every different number — is a variation on that sentence: an allowance raised, reduced, removed, or taxed on a different basis. HMRC sets your code and sends it to your employer; payroll cannot invent or change it, only apply it.
The letters, decoded
| Code | Meaning | Who gets it |
|---|---|---|
| 1257L | Standard personal allowance (£12,570) | Most employees, one job, no complications |
| 1131M | Allowance reduced by £1,260 | You transferred 10% of your allowance to your spouse (Marriage Allowance giver) |
| 1383N | Allowance raised by £1,260 | You received your partner’s transferred allowance (Marriage Allowance receiver) |
| S / C prefix | Scottish / Welsh tax rates apply | Scottish (S1257L) and Welsh (C1257L) taxpayers — set by where you live, not where you work |
| BR | Everything taxed at 20%, no allowance | Second jobs and pensions — very common on NHS bank-only contracts |
| D0 / D1 | Everything taxed at 40% / 45% | Second incomes of higher/additional-rate taxpayers |
| 0T | No allowance at all | New starter with no P45 and no starter checklist, or allowance exhausted elsewhere |
| K475 | A negative allowance: £4,750 is added to taxable pay | Tax owed exceeds your allowance — company benefits, state pension, or collected underpayments |
| W1 / M1 / X | Emergency basis: each payday taxed in isolation | Temporary — new job, changed payroll, or HMRC catching up; marked “Wk1/Mth1” on the payslip |
| NT | No tax deducted | Rare — instructed by HMRC in specific circumstances |
Two of these deserve a closer look because they frighten people unnecessarily. A K code does not mean something has gone wrong: it means HMRC is collecting tax on income or benefits worth more than your allowance — the state pension paid without tax deducted is the classic cause, and for working NHS staff a lease car or private medical benefit can do it. And an emergency W1/M1 code almost always resolves itself within a month or two once HMRC matches your records; it taxes each pay period as if it were the first, which over-deducts when pay is uneven — bank and overtime-heavy months suffer most — and the difference is corrected automatically once the cumulative code lands, or by refund after the year end.
Why your tax code changed — the five usual causes
1. An underpayment is being collected. If you underpaid tax in an earlier year, HMRC usually collects it through your code rather than sending a bill: the allowance is cut by the debt divided by your tax rate. Owe £240 as a basic-rate taxpayer and your allowance falls by £1,200 — 1257L becomes 1137L — costing exactly £20 a month for a year. 2. A taxable benefit appeared — a lease car, private medical cover, or similar — and its value is carved out of your allowance. 3. Your state pension started: it is paid gross, so HMRC collects the tax through the code on your job or occupational pension instead. 4. You started or left a job and the paperwork is mid-flight — expect 0T or W1/M1 briefly. 5. You claimed or received Marriage Allowance, moving £1,260 of allowance between you and your partner and switching the letters to M and N. In every case the letter from HMRC (your Tax Code Notice) explains which one it was: it is the most ignored useful letter in British life.
NHS-specific code traps
NHS pay patterns collide with the code system in predictable places. Bank-only and second assignments are normally coded BR, because your personal allowance is used by your main job — correct, not an error, and reconciled by HMRC after the year ends if too much was taken overall. Doctors and senior staff with several employers (trust, university, private practice) often hold a different code against each income, and only one gets the allowance. Staff near £100,000 lose personal allowance at £1 for every £2 earned over the threshold, so Band 8d and Band 9 staff and consultants see codes shrink — sometimes to 0T with a K code — as estimates of total income move. Scottish taxpayers working for English trusts keep their S code: the code follows your home address, and the S matters — Scottish rates and bands differ from England’s, as our Scotland calculator shows. And if your code looks wrong, the fix is never payroll: only HMRC can change a code, via your personal tax account or by phone, and payroll must apply the new code from the next pay run once HMRC issues it. Our payslip guide shows where the code sits and how it flows into the tax line.
Tax code questions
What does 1257L mean?
It is the standard UK tax code for 2026/27. The number 1257, multiplied by ten, is your tax-free personal allowance: £12,570 a year. The letter L means the ordinary allowance applies with nothing added or removed. If your payslip shows 1257L and you have one job and no taxable benefits or debts being collected, your tax should be exactly what our calculators show.
Why has my tax code gone down?
A lower number means a smaller tax-free allowance, so more of your pay is taxed and take-home falls. The usual causes are HMRC collecting an earlier underpayment through the code, a taxable benefit (car, medical insurance) being added, your state pension starting, or your income passing £100,000 and the allowance being tapered away. Your Tax Code Notice from HMRC states the reason — if it is wrong, only HMRC can fix it, not payroll.
What is a K code? Is it bad?
A K code is a negative allowance: instead of shielding income from tax, it adds an amount to your taxable pay — K475 adds £4,750. It usually means you receive income that arrives untaxed (most often the state pension) or benefits worth more than your remaining allowance, and HMRC collects the tax through your wages or occupational pension. It is an accounting mechanism, not a penalty — but it is worth checking the figure on your code notice, because an overstated K code over-collects every month.
My bank shifts are taxed at BR. Is that wrong?
Almost certainly not. BR means the income is taxed at flat 20% with no personal allowance, which is the standard treatment for a second job or second payroll assignment, because your allowance is already used by your main job. If your total income across both jobs ends up over-taxed — common if the bank work is occasional — HMRC reconciles it after the tax year and refunds the difference (the P800 calculation). If the bank work is your only income, tell HMRC: your allowance should move to it.
Who do I contact about a wrong tax code?
HMRC, not your employer. Payroll is legally required to apply the code HMRC issues and cannot change it, however obviously wrong it looks. Check the code and the figures behind it in your HMRC personal tax account (or the HMRC app), where you can also report changed circumstances — a benefit stopped, a second job ended. HMRC then issues a corrected code to your employer, which applies from the next pay run, and cumulative tax self-corrects within the year.