What is the difference between W1, M1 and X?

W1 is applied to weekly payrolls, M1 to monthly. X is a generic non-cumulative suffix used for any other frequency (4-weekly, fortnightly, daily) or when payroll software does not differentiate. Functionally identical - all three strip the cumulative calculation.

Why is "0T non cumulative tax code" so common?

Because it is the code gov.uk tells employers to use when a new starter gives neither a P45 nor a Starter Checklist. This combination strips both the Personal Allowance AND the cumulative calculation - the most punitive standard combination - until HMRC issues a corrective code. Handing over a P45, or completing a Starter Checklist, on day one avoids it.

Will HMRC refund tax automatically?

Yes - once HMRC moves you to a cumulative code (no W1/M1/X suffix), the first cumulative payslip rebalances year-to-date totals and any over-paid tax flows back through that payslip. If the year ends first, the automatic year-end reconciliation produces a P800 with refund instructions; gov.uk says these letters are sent between June and March of the following tax year.

Does W1/M1 affect my pension contributions?

No - W1/M1 affects only Income Tax PAYE calculation. Pension contributions (workplace auto-enrolment, salary sacrifice, relief at source) and National Insurance use their own calculation rules that are unaffected by the cumulative/non-cumulative basis of Income Tax.

Tax Code · 2026/27

W1 / M1 / X Tax Code Suffix: Non-Cumulative PAYE

W1, M1 and X suffixes mean your PAYE is calculated on a non-cumulative basis - each pay period treated in isolation, ignoring year-to-date earnings. You get 1/52 or 1/12 of your Personal Allowance each period.

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What the W1 / M1 / X tax code means

A regular cumulative tax code (like 1257L on its own) calculates tax based on total earnings since 6 April. PAYE looks at how much PA and basic-rate band you have used year-to-date and corrects in each payslip.

Adding W1, M1 or X strips that cumulative calculation: each payslip is treated as a standalone tax event. You get 1/12 of the PA in a monthly payslip (£1,047.50) or 1/52 in a weekly one (£241.73), and your basic rate band is similarly sliced.

HMRC's PAYE Manual (PAYE11090) says the aim of the week 1/month 1 basis is to prevent the employer making heavy deductions or giving any refund. It is the starter basis for some new employees (Statement B on the Starter Checklist, or no declaration at all), and HMRC uses it when it lowers a code partway through the year, so the extra tax is collected over the remaining months rather than all at once (PAYE11120).

When you'll see W1 / M1 / X

You started a new job without a P45 and ticked Statement B on the Starter Checklist, or gave no Starter Checklist at all (0T W1/M1). A starter who hands over a P45 normally gets the code on the P45, operated cumulatively unless the P45 itself is marked week 1/month 1.
You returned to PAYE after a period out of work (maternity, sabbatical, self-employed) and HMRC is rebuilding your year-to-date position.
HMRC lowers your code partway through the year - a revised code lower than the last cumulative one is issued on a week 1/month 1 basis, so the extra tax is spread over the remaining pay periods instead of being taken in one large deduction (PAYE11120).
Pension drawdown or one-off taxable lump sums - providers often apply emergency M1 on first payment.

What to do if you have a W1 / M1 / X code

For most new starters: provide your P45 from the previous employer immediately. Otherwise HMRC sends a code once it has details from your new and previous employers, which gov.uk says can take up to 35 days from when you start.
Check your Personal Tax Account (gov.uk/personal-tax-account) - look for the tax code notice explaining why non-cumulative basis is applied.
If you think the year-to-date figures are wrong or the W1/M1 is unjustified, call HMRC on 0300 200 3300 with your NI number.
Any over-tax under W1/M1 is normally refunded automatically through subsequent payslips once HMRC moves you to cumulative basis, or via a P800 reconciliation after year-end.

Worked example

On 1257L M1 at £4,000 monthly gross: only 1/12 of the allowance applies (£1,047.50), leaving £2,952.50 taxable, so tax at 20% is £590.50 a month. On cumulative 1257L at the same £4,000 a month, by month 6 your pay to date is £24,000 and six months of allowance (£6,285) have been given, leaving £17,715 taxable and £3,543 of tax to date - the same £590.50 a month. The two bases only differ when pay to date is uneven: a mid-year start, a large bonus, or a return from unpaid leave.

Want to see the numbers for your own salary? Use the salary calculator and pick 2026/27 to see how W1 / M1 / X interacts with your full take-home.

Frequently asked questions about W1 / M1 / X

What is the difference between W1, M1 and X?: W1 is applied to weekly payrolls, M1 to monthly. X is a generic non-cumulative suffix used for any other frequency (4-weekly, fortnightly, daily) or when payroll software does not differentiate. Functionally identical - all three strip the cumulative calculation.
Why is "0T non cumulative tax code" so common?: Because it is the code gov.uk tells employers to use when a new starter gives neither a P45 nor a Starter Checklist. This combination strips both the Personal Allowance AND the cumulative calculation - the most punitive standard combination - until HMRC issues a corrective code. Handing over a P45, or completing a Starter Checklist, on day one avoids it.
Will HMRC refund tax automatically?: Yes - once HMRC moves you to a cumulative code (no W1/M1/X suffix), the first cumulative payslip rebalances year-to-date totals and any over-paid tax flows back through that payslip. If the year ends first, the automatic year-end reconciliation produces a P800 with refund instructions; gov.uk says these letters are sent between June and March of the following tax year.
Does W1/M1 affect my pension contributions?: No - W1/M1 affects only Income Tax PAYE calculation. Pension contributions (workplace auto-enrolment, salary sacrifice, relief at source) and National Insurance use their own calculation rules that are unaffected by the cumulative/non-cumulative basis of Income Tax.

All UK tax codes →

Sources & further reading

All figures and definitions on this page reflect the 2026/27 UK tax year and are cross-checked against HMRC guidance.

HMRC — What your tax code means (official meanings for L, BR, D0, D1, K, 0T, NT, M, N, S, C, T)
HMRC — Income Tax rates and Personal Allowance (2026/27 bands and thresholds)
HMRC — Emergency tax codes (W1, M1 and X suffixes)
HMRC — Rates and thresholds for employers 2026 to 2027
HMRC — Your Personal Tax Account (check your current code and how it was calculated)
Our methodology & calculation sources →