Written and reviewed by James Whitfield · Updated for 2026/27 · Editorial standards · Methodology
Why emergency tax codes appear, how much extra you pay, and how to get HMRC to correct the code quickly.
An emergency tax code, usually 1257L W1 or M1 (sometimes shown as 1257L X), gives you one week's or one month's personal allowance each payday and ignores the rest of the tax year so far. It is used when a new employer has no P45. On £3,000 a month it takes £390.50 tax, the same as a normal month, but you lose any allowance built up while you were not working. HMRC fixes it when you update your details in your personal tax account; the overpaid tax then comes back on your next payslip, or by a P800 after 5 April.
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Typical default outputs for quick context.
An emergency tax code is a temporary code your employer applies when HMRC cannot confirm your correct tax position, usually because you have started a new job without providing a P45, or because HMRC does not yet hold information about your income. The most common emergency code is 1257L W1/M1. The '1257L' part means your personal allowance is treated as £12,570, which is correct, but the W1 (week 1) or M1 (month 1) suffix means you are taxed on a non-cumulative basis.
Non-cumulative means the payroll system ignores any tax you have already paid in the current tax year. Instead of spreading the annual allowance and thresholds across the whole year and catching up on any previous underpayment or overpayment, it treats each pay period as if it is the first of the year. So if you start a new job in month 8 having earned little or nothing since April, W1/M1 gives you only one month's allowance (£1,047.50) per payday instead of the eight months you have built up, and you overpay until the code is made cumulative. Codes that give no personal allowance at all (0T) over-deduct even more.
The '0T' code is particularly harsh. It applies no personal allowance at all and taxes all earnings from pound one. On a £3,000 monthly salary, that means income tax of £600 per month instead of £390.50 on 1257L, about £210 a month too much. This is the code most likely to cause the dramatic over-deductions people report when starting a new job.
The most common trigger is starting a new job without handing over a P45. Your P45 is a form your previous employer gives you when you leave. It tells HMRC and your new employer what you earned and how much tax you paid in the current tax year, which allows your new employer to calculate cumulative tax correctly from day one.
Students starting their first job, people returning to work after a break, and anyone who lost their P45 are the most common casualties. Agency workers are also frequently affected, especially on short-term assignments where paperwork moves slowly. The system automatically generates emergency codes rather than guessing, which is the right policy, it just results in over-deduction for the employee until the position is corrected.
If you are on more than one job simultaneously, your secondary employer is supposed to use a BR or D0 code (taxing at 20% or 40% with no allowances) because your personal allowance should be claimed on the primary job. Emergency coding on a second job is therefore less of a problem, but it does mean you need to ensure your primary employer is applying the allowance correctly.
The fastest route is to contact HMRC directly. You can do this via your Personal Tax Account at gov.uk, through the HMRC app, or by calling the income tax helpline on 0300 200 3300. You will need your National Insurance number, your payroll number or PAYE reference, and details of your current employer. HMRC will update your tax code and issue a new one to your employer, usually within a few days.
Once the correct code is applied, most payroll systems will automatically calculate any overpaid tax from earlier in the year and refund it through your pay, this happens automatically in the same tax year because cumulative PAYE catches up. If the code is not corrected before the tax year ends on 5 April, HMRC's year-end check sends you a P800 tax calculation and the refund follows. If you have stopped working altogether, you can claim straight away with form P50.
If you cannot reach HMRC and you need to give your employer something to work with, you can fill in a 'Starter Checklist' (formerly called a P46) which your employer uses to apply an interim code. It asks whether this is your only job and whether you have been in employment recently, the answers determine which code gets applied before HMRC confirms the right one.
Current tax-year thresholds used across this guide and calculator.
A temporary code, usually 1257L W1, M1 or X, that taxes each payday on its own without looking at earlier pay in the tax year. Employers use it when they do not have a P45 or HMRC has not yet sent the right code.
Until HMRC sends your employer a cumulative code. Updating your employment details in your HMRC personal tax account is usually the quickest way; the new code normally reaches payroll within one or two pay periods.
Yes, if you overpaid. Once the code is cumulative, your next payslip refunds the overpayment for the year so far. If it is never corrected, HMRC's P800 after the tax year ends refunds it.
0T gives no personal allowance at all, so every pound is taxed. It is used when HMRC or your employer has no information to give you an allowance. On £3,000 a month it takes £600 tax instead of £390.50.
No. National Insurance is always worked out pay period by pay period, so a tax code change does not alter it.
Yes. Use the scenario links in this guide to open prefilled states, then adjust salary, region, loan and pension settings.
The rates, thresholds and rules on this page are drawn from the official UK government sources below, using the confirmed 2026/27 figures. Each link opens the relevant HMRC or GOV.UK page in a new tab.
Verified against published UK government guidance for 2026/27.