Answers · UK 2025/26
What does a week 1 / month 1 tax code mean and will I get money back?
A week 1 / month 1 code (shown as W1, M1 or X after the number) tells your employer to tax each pay period in isolation, ignoring earlier pay and tax in the year. It is usually temporary after a job change. You often overpay and get a refund automatically once HMRC issues a cumulative code or after the tax year ends.
Full answer
Most tax codes are cumulative: your employer looks at total pay and tax-free allowance used so far this year, smoothing deductions. A week 1 / month 1 code is non-cumulative - each pay packet is taxed as if it were the first of the year, giving you 1/12 of your allowance that month only and ignoring what came before. HMRC applies it when it lacks full information, for example after starting a new job without a P45, or after a coding change. It prevents a sudden large refund or underpayment mid-year but can leave you over- or under-taxed. Worked example: Jess starts a new job in August 2026 on code 1257L M1. Her standard tax-free Personal Allowance is GBP 12,570 a year, or GBP 1,047.50 a month. Because the code is non-cumulative, she gets only that monthly slice and cannot reclaim the unused allowance from April to July through payroll. By year end she may have overpaid; HMRC reconciles this after 5 April 2027 and issues a P800 refund, or sooner if it switches her to a cumulative 1257L code. To speed things up, give your employer your P45 or complete the new starter checklist so HMRC can move you onto a cumulative code. The take-home pay calculator shows the difference between cumulative and emergency taxation on your salary. Check your code and report changes at gov.uk/tax-codes.
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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.