Answers · UK 2025/26
What does a Week 1/Month 1 tax code mean on my payslip?
A Week 1/Month 1 (W1/M1) code taxes each pay period in isolation rather than cumulatively. It still gives you a slice of your Personal Allowance each period - 1/12 of GBP 12,570 (GBP 1,047.50) monthly - but ignores earlier months, so HMRC cannot refund or reclaim earlier over- or under-payments through your pay.
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A normal tax code (such as 1257L) is cumulative: each payday your employer looks at your total pay and total tax-free allowance for the year so far, so any over- or under-payment self-corrects across the year. A Week 1/Month 1 code, shown as 1257L W1 or 1257L M1, switches that off. Each pay period is taxed on its own, using just one period's worth of allowance: monthly that is GBP 12,570 / 12 = GBP 1,047.50 tax-free, with 20% on the next GBP 3,119 (GBP 37,700 / 12) and 40% above. Example: you earn GBP 3,500 in a month on an M1 code. Tax-free GBP 1,047.50; basic rate on GBP 2,452.50 at 20% = GBP 490.50. You pay the same headline tax as a cumulative code would in a steady month, but if you had unused allowance from earlier months (say you started mid-year), an M1 code will NOT refund it - you keep overpaying until HMRC issues a cumulative code or reconciles after year end. HMRC commonly applies W1/M1 codes after a job change, an emergency code, or a mid-year code change to stop a large one-off adjustment. Once HMRC has full pay details it usually reverts you to a cumulative code and any overpaid tax is returned through payroll or via a P800. Model your monthly figures with the Take-Home Pay calculator, and check your current code in your Personal Tax Account at gov.uk/check-income-tax-current-year.
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This answer is informational only and does not constitute financial, tax or legal advice. Figures are for the 2025/26 UK tax year. See our methodology and sources.