Why am I on an emergency tax code?

Almost always because HMRC is missing year-to-date information from a previous employer or tax period. Submitting a P45 or Starter Checklist resolves it within 1-2 pay cycles.

Does emergency code mean I'm paying more tax?

In many cases yes, temporarily. Non-cumulative calculation doesn't account for unused PA from earlier months, so you may be over-taxed until the code goes cumulative. Any excess is refunded via subsequent payrolls or year-end reconciliation.

What's the difference between W1, M1 and X?

W1 is weekly (used if you're paid weekly), M1 is monthly, and X is a generic non-cumulative marker that can apply to any frequency. Functionally they operate identically — taxing each period in isolation.

Will I get a refund if emergency code over-taxed me?

Yes. Once HMRC converts you to a cumulative code (typically 1257L without the W1/M1/X suffix), the first cumulative payroll adjusts your year-to-date figures and refunds any over-paid tax through that payslip. You don't need to file self-assessment for this alone.

Emergency Tax Code — W1, M1, X (UK 2026/27)

What the 1257L W1/M1/X tax code means

Under normal PAYE, your employer calculates tax using year-to-date pay and allowance totals — this 'cumulative' approach smooths out any under- or over-collection across the year.

An emergency code (W1, M1 or X suffix) switches to non-cumulative — each pay period is taxed in isolation, as if it were the first week or month of the tax year. You get 1/52nd or 1/12th of your Personal Allowance applied, with no year-to-date adjustment.

The code itself is usually 1257L W1 (weekly) or 1257L M1 (monthly). The W1/M1/X signal that HMRC hasn't yet received your full year-to-date information from a previous employer or pension.

When you'll see 1257L W1/M1/X

You've just started a new job and haven't provided a P45 from your previous employer.
You completed a Starter Checklist but HMRC hasn't yet matched it to your full year record.
You had gaps in employment and HMRC is playing catch-up on year-to-date figures.
You moved from self-employment to employment mid-year.

What to do if you have a 1257L W1/M1/X code

Submit your P45 or Starter Checklist to your new employer at the earliest opportunity — this is the fastest path to a full cumulative code.
Check your Personal Tax Account on gov.uk to see the code HMRC has on file and the reason for non-cumulative handling.
Once HMRC has the full year-to-date, they'll remove the suffix and issue a standard 1257L code. Any over- or under-tax from the emergency period is reconciled through the next payroll.
If emergency coding lasts more than 2 months, contact HMRC on 0300 200 3300 — they can manually push the full code to your employer.

Worked example

You start a new job paying £35,000 on 1st June with no P45 submitted. Your employer applies 1257L M1. In June, only 1/12th of the PA (£1,047.50) is considered, and only June's pay is taxed against that slice — no carry-forward from the April-May period when you were self-employed. This usually over-taxes you initially; once the full P45 arrives, HMRC issues 1257L (cumulative) and the next payroll refunds the excess.

Want to see the numbers for your own salary? Use the salary calculator and pick 2026/27 to see how 1257L W1/M1/X interacts with your full take-home.

Frequently asked questions about 1257L W1/M1/X

Why am I on an emergency tax code?: Almost always because HMRC is missing year-to-date information from a previous employer or tax period. Submitting a P45 or Starter Checklist resolves it within 1-2 pay cycles.
Does emergency code mean I'm paying more tax?: In many cases yes, temporarily. Non-cumulative calculation doesn't account for unused PA from earlier months, so you may be over-taxed until the code goes cumulative. Any excess is refunded via subsequent payrolls or year-end reconciliation.
What's the difference between W1, M1 and X?: W1 is weekly (used if you're paid weekly), M1 is monthly, and X is a generic non-cumulative marker that can apply to any frequency. Functionally they operate identically — taxing each period in isolation.
Will I get a refund if emergency code over-taxed me?: Yes. Once HMRC converts you to a cumulative code (typically 1257L without the W1/M1/X suffix), the first cumulative payroll adjusts your year-to-date figures and refunds any over-paid tax through that payslip. You don't need to file self-assessment for this alone.

All UK tax codes →

Sources & further reading

All figures and definitions on this page reflect the 2026/27 UK tax year and are cross-checked against HMRC guidance.

HMRC — What your tax code means (official meanings for L, BR, D0, D1, K, 0T, NT, M, N, S, C, T)
HMRC — Income Tax rates and Personal Allowance (2026/27 bands and thresholds)
HMRC — Emergency tax codes (W1, M1 and X suffixes)
HMRC — Rates and thresholds for employers 2026 to 2027
HMRC — Your Personal Tax Account (check your current code and how it was calculated)
Our methodology & calculation sources →