Week 1 / Month 1 Basis

Week 1/Month 1 (also called 'X basis' on some payroll software) is a non-cumulative method of running PAYE. Instead of looking at year-to-date pay and tax, the payroll system gives you 1/52 (or 1/12) of your personal allowance each period and taxes the rest at the appropriate rates. Earlier periods' under- or over-payments are not corrected; each pay period stands alone.

W1/M1 is signalled by a tax code suffix: 1257L W1, 1257L M1 or 1257L X. HMRC issues it as a protective measure when there is uncertainty about the employee's total pay for the year — typically after a P45 starter where the previous job had a non-standard tax position, or where HMRC detects a possible over-payment that would otherwise compound under cumulative PAYE.

Worked example: Rosa starts a new job in November 2026 on tax code 1257L M1. Her monthly pay is £4,000. The payroll system gives her £1,047.50 of monthly tax-free allowance and taxes the remaining £2,952.50 at 20% = £590.50 each month. Her tax is identical every month from November onwards, regardless of the fact that she earned only £8,000 in the first half of the year. At year-end she is likely to have overpaid (because she didn't use her full personal allowance for April–October) and HMRC will issue a P800 in the summer of 2027 with a refund. Cumulative codes self-correct in-year; W1/M1 codes only self-correct after year-end. If you spot a W1, M1 or X suffix on your tax code and you have been at the same job for several months, ringing HMRC to switch to a cumulative basis usually triggers a refund in the next pay run.