Answers · UK 2025/26

What does the emergency tax code 1257L W1/M1 mean?

1257L W1/M1 is an emergency tax code that gives you the standard £12,570 Personal Allowance but applies it non-cumulatively -- each pay period is taxed in isolation rather than based on your total earnings and tax paid so far in the tax year. It usually appears when you start a new job without a P45, and often needs correcting to avoid over- or under-paying tax.

Last updated: 1 July 2026Last reviewed: 1 July 2026How we verify

Full answer

The tax code 1257L is the standard code for 2026/27 reflecting the full £12,570 Personal Allowance, applied at the standard basic/higher/additional rate bands. The "W1" (week 1) or "M1" (month 1) suffix changes HOW that allowance is applied, not how much allowance you get. **Cumulative vs non-cumulative (emergency) basis** Under the normal CUMULATIVE basis, each payslip looks at your total pay and tax paid so far in the tax year (since 6 April) and works out the correct cumulative tax due, correcting any previous under- or over-deduction automatically as the year progresses. Under the W1/M1 (week 1/month 1) NON-CUMULATIVE basis, each pay period is treated in isolation as if it were the only pay period in the year -- your allowance is simply divided by 52 (weekly) or 12 (monthly) and applied fresh each time, with no reference to earlier pay periods. **When W1/M1 is used** This emergency basis is typically applied when: - You start a new job without providing a P45 from your previous employer - HMRC does not yet have enough information to issue you a normal cumulative code - You have had a significant change in circumstances mid-year that HMRC has not yet processed **Why it can cause over- or under-payment** Because each period is calculated in isolation, if you had unused Personal Allowance from earlier in the tax year (e.g. you were unemployed for the first three months), a W1/M1 code will NOT automatically give you credit for that unused allowance -- you could pay more tax than you should in the short term. Conversely, if you had already used significant allowance elsewhere, W1/M1 might initially undertax you. **Worked example** James starts a new job in October without a P45 (having been unemployed since April). Under a normal cumulative code, his six months of unused Personal Allowance (roughly £6,285) would reduce his October tax bill significantly. Under 1257L W1/M1, October is taxed as if it were a standalone month with only 1/12th of the allowance (£1,047.50), so James is likely to overpay tax for that month. **How it gets fixed** Once HMRC receives your P45 details or processes new starter information (via the P46/starter checklist), they typically issue a corrected cumulative tax code, and any overpayment is usually refunded through payroll automatically, or you can claim it back directly from HMRC if the job ends before correction happens.