What tax code W1 / M1 / X actually means
Suffixes, not codes: W1 (weekly), M1 (monthly) and X switch off the year-to-date calculation.
W1, M1 and X aren't tax codes by themselves — they're suffixes that make whatever code you have non-cumulative. Each payday is treated as if it were week 1 or month 1 of the year: one period's slice of allowance (£1,047.50 a month on 1257L), one period's slice of each band, and no correction for what's already happened this year. The tax rate is the same; the self-correcting maths is what's missing.
- Personal allowance
- Your code's allowance, one period-slice at a time
- Applies in
- All UK regions
- Calculation
- Non-cumulative by definition
Non-cumulative code — results assume consistent weekly earnings
See this salary on a different tax code → (opens the main calculator pre-filled with W1 / M1 / X)
What does W1 / M1 / X mean?
Normal PAYE is cumulative: every payday, payroll recomputes the whole year — total pay to date, total allowance to date, total tax due to date — and deducts the difference. That running reconciliation is why a mid-year pay change or job gap sorts itself out on the next payslip.
Add W1 (paid weekly), M1 (paid monthly) or X (either) and that machinery stops. Payroll sees only the current period: one twelfth of the allowance, one twelfth of the basic-rate band, tax on this month's pay alone. Steady earner? The result matches cumulative to the penny. Anything irregular — a gap, a bonus, a late start — and errors accumulate that the code can't fix, because it never looks back.
HMRC uses non-cumulative codes deliberately when the year-to-date picture is unknown or would cause a problem — new starters without a P45, or mid-year code reductions where back-calculating would swallow a whole payslip. It's meant as a temporary state; a cumulative code should follow.
When you’ll see W1 / M1 / X
- New job without a P45 — the classic emergency-tax scenario.
- HMRC cut your code mid-year and applied it W1/M1 so arrears aren't clawed back in one painful payslip.
- A pension provider making first payments before HMRC confirms a code.
- A payment made after you've left a job (0T W1/M1 by rule).
W1 / M1 / X vs other common codes
| Code | Personal allowance | How income is taxed |
|---|---|---|
| 1257L | £12,570 | Standard bands after the allowance |
| BR | £0 | Flat 20% on everything |
| 0T | £0 | Normal bands from the first pound |
| D0 | £0 | Flat 40% on everything |
W1 / M1 / X questions, answered
Related tax codes
1257L
The standard code for 2025/26 — a £12,570 tax-free personal allowance, applied evenly across the year.
Decode itEmergency tax (W1/M1/X)
Usually a 1257L code with a W1, M1 or X suffix — each payday taxed in isolation until HMRC confirms your details.
Decode itBR
Basic Rate on everything: 20% from the first pound, no personal allowance — normal for second jobs and pensions.
Decode it0T
Zero allowance, but normal bands: 20%, then 40%, then 45% — from the very first pound you earn.
Decode itScottish tax codes (S)
An S at the front of your code means Scottish rates: six bands from 19% to 48% in 2025/26.
Decode itK codes
The code that works in reverse: instead of tax-free pay, a K code adds to your taxable income.
Decode itNot sure this is the right code for you?
Don’t take our word for it — HMRC holds the code they’ve actually issued for each of your jobs, and you can check it in two minutes.
Sources: tax codes — gov.uk · income tax rates · Scottish income tax