Tax Code Calculator (1257L explained)

What a tax code is — and where to find yours

A tax code is the short string of numbers and letters — 1257L for most people in 2026/27 — that tells your employer or pension provider how much Income Tax to deduct from each payslip under PAYE. HMRC issues it; your employer just applies it. Get the code wrong and every payslip is wrong, which is why a two-minute check can be worth hundreds of pounds a year.

You can find your tax code in four places: on any payslip (usually near your National Insurance number), on the P45 your last employer gave you when you left, on the P60 summary issued after each 5 April, and — the most up-to-date source — in the HMRC app or your online Personal Tax Account, which also shows how HMRC calculated it and lets you report a change instantly.

The code only controls *how* tax is spread across the year, not *how much* tax you ultimately owe. But a wrong code means you overpay (and wait for a refund) or underpay (and get a bill later), so checking it against your circumstances matters. Once you know your code is right, see what actually lands in your bank with the take-home pay calculator.

1257L decoded: the maths behind the most common code

The recipe is simple: take the number, multiply by 10, and that is your tax-free Personal Allowance. 1257 × 10 = £12,570, the standard allowance for 2026/27 (frozen at this level since 2021). The L letter means you qualify for that standard allowance with no adjustments — no Marriage Allowance, no benefits-in-kind clawback, no untaxed income.

PAYE spreads the allowance evenly across the year. On a monthly payroll you get £1,047.50 of tax-free pay each month (£12,570 ÷ 12); on a weekly payroll, £241.73 a week. Everything above that slice is taxed at 20% up to £50,270 of annual income, 40% up to £125,140, and 45% beyond — with the allowance itself tapering away between £100,000 and £125,140 at £1 lost per £2 earned.

Because 1257L is normally cumulative, each payslip looks at your total pay and total allowance for the year so far and corrects any earlier over- or under-deduction automatically. That is why a bonus month can look heavily taxed and the next month lighter — the code is rebalancing. Run the band-by-band numbers with the income tax calculator.

Every tax code letter, decoded

The letters are where tax codes get cryptic. Here is the complete GOV.UK list for 2026/27 — meaning first, then who typically gets it. Prefixes come before the number (S1257L, C1257L, K475); suffix letters come after it (1257L, 1383M).

Emergency tax: what 1257L W1/M1 does to your pay

An emergency code — 1257L W1, 1257L M1 or 1257L X — appears when your new employer does not have enough information to place you cumulatively, most often because you started a job without a P45 (first UK job, returning from abroad, or moving straight from self-employment). HMRC also uses it after big mid-year changes to company benefits.

On W1/M1 you still get the standard allowance, but only one week’s or one month’s slice at a time, and each payslip is taxed as if it were the only one in the year. The code ignores unused allowance from earlier months — so someone who starts work in October having earned nothing since April cannot use the six months of allowance they have already accrued, and pays roughly £290.50 a month more on a £30,000 salary than a cumulative code would charge until it is fixed.

The good news: emergency tax usually self-corrects. Once HMRC receives your details (via your employer’s payroll submission or the starter checklist), it issues a cumulative 1257L, and the next payslip refunds any overpayment automatically. If the tax year ends before that happens, HMRC reconciles it through a P800 calculation and refunds you — but you can speed things up through your Personal Tax Account rather than waiting.

K codes: when your allowance runs out — and then some

A K code turns the allowance negative. It appears when your untaxed income — company car benefit, employer-paid medical insurance, the state pension paid alongside a job or private pension, or old tax debts collected through PAYE — is worth more than your £12,570 Personal Allowance. Instead of subtracting tax-free pay, payroll *adds* notional pay before calculating tax.

Worked example — K475: suppose your company car and fuel benefit are worth £17,320 a year. That exceeds your £12,570 allowance by £4,750, so HMRC issues K475 (4,750 ÷ 10, rounded). On a £30,000 salary, payroll taxes you as if you earned £34,750: 20% × £34,750 = £6,950 a year (£579.17 a month) — £3,464 more than plain 1257L, which is precisely the tax due on the car benefit.

One protection applies: a K code can never take more than 50% of your gross pay in tax on any payslip. Anything above that limit rolls forward. If a K code appears and you no longer have the benefit that caused it (returned the company car, cleared the debt), tell HMRC — the code does not fix itself.

Why your code might not be 1257L — the usual suspects

Around one in three UK taxpayers is on something other than plain 1257L, and often correctly so. Before assuming an error, check whether one of these applies to you:

• A second job or pension (BR, D0, D1, 0T). Your allowance is applied once, against your main income; the second source is taxed flat. Correct in principle — but if the *wrong* job carries the allowance, or your main job pays under £12,570, you overpay.
• Benefits in kind (lower number or K code). Company car, private medical cover or a low-interest loan reduce the number — e.g. medical insurance worth £1,200 turns 1257L into 1137L. Check the benefit values match your P11D or payrolled-benefit figures.
• Marriage Allowance (M or N). Transferring 10% of the allowance (£1,260) produces 1383M for the recipient and 1131N for the giver — worth up to £252 a year. Check eligibility with the marriage allowance calculator.
• Untaxed interest or side income. HMRC often collects tax on savings interest or small freelance income by shrinking your code rather than issuing a bill.
• Under- or over-payments from earlier years. Debts are collected by reducing the number; some refunds are given by increasing it.
• A taper above £100,000 (T or 0T). The allowance shrinks by £1 per £2 over £100,000 and disappears at £125,140.
• Plain error. Employer typos, duplicated employments after a job change, or HMRC estimating income wrongly. This is common in the first months after switching jobs.

How to fix a wrong tax code — and claim money back

You cannot change your own tax code through payroll — only HMRC can issue a new one. The fastest route is the Personal Tax Account (or the HMRC app): under “Pay As You Earn” you can see every employment HMRC thinks you have, correct estimated income, remove dead employments and report missing benefits. Most code changes triggered online take effect on the next payroll run.

Prefer the phone? Call HMRC on 0300 200 3300 (Income Tax enquiries, Monday–Friday), with your National Insurance number to hand. If you have overpaid in the *current* year, the corrected cumulative code refunds you automatically through your next payslip — no claim form needed.

For past tax years, HMRC runs an automatic reconciliation after each 5 April and sends a P800 letter (or a PA302 Simple Assessment) if you overpaid — you claim the refund online in minutes, or a cheque follows if you do nothing. You can go back four tax years: in 2026/27 that means claims for 2022/23 are still open, but only until 5 April 2027. If you have never checked your codes after years of job switches, that deadline is the reason to do it now.

Worked examples: what the code does to real payslips

Three scenarios show why the code matters more than most people think. All use 2026/27 rates for England, Wales and Northern Ireland; National Insurance (checked with the national insurance calculator) is unaffected by tax codes and ignored here.