Glossary · UK
What is Week 1 / Month 1 Basis?
Non-cumulative PAYE method that taxes each pay period in isolation, ignoring year-to-date totals.
Full Definition
Week 1 / Month 1 (sometimes shown as W1, M1, or X after the tax code, e.g. 1257L W1) is a non-cumulative PAYE basis. Each pay period is taxed using one week's or one month's allowance only — no year-to-date true-up. HMRC commonly issues a W1/M1 code when starting a new job without a P45, or after a coding change mid-year to prevent a sudden tax adjustment. The downside is that genuine over- or under-payments do not self-correct until HMRC reconciles the year — typically via a P800 the following summer. You can ask HMRC to switch to a cumulative basis once they have full year-to-date pay history.
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Disclaimer: Definitions are for guidance only. For decisions about your tax, savings, property or pension situation, always consult a qualified professional or refer to gov.uk.