Tax Code M1 / W1 / X Explained

M1 (Month 1), W1 (Week 1) and X after a tax code mean the code is non-cumulative. Each payday is taxed as if it were the first of the tax year, ignoring pay and tax earlier in the year. GOV.UK treats these as emergency tax codes. X is used for irregular pay dates; you may also see 'NONCUM'.

How your pay is taxed on M1 / W1 / X

Normal (cumulative) codes compare your total pay so far this year with your total allowance so far, so they automatically refund overpayments or catch up on underpayments. Non-cumulative codes don't. Example: on 1257L M1 you get £1,047.50 tax-free every month no matter what. If you earned nothing April–August and start on £2,500 a month in September, a cumulative 1257L would let you use five months of unused allowance (£5,237.50) and pay no tax in September; 1257L M1 charges £290.50 in September and every month after.

Tax-free allowance from this code£12,570Cumulative

Income tax with this code	£3,486
Income tax with standard 1257L	£3,486
Difference per month	£0.00

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Why you might have this code

You started a new job without a P45 (or ticked Statement B on the starter checklist).
HMRC changed your code late in the year and wants it applied from now on only.
You started a new pension or taxable benefit.

What to do

Check the code HMRC holds for you in your personal tax account (GOV.UK sign-in) or the HMRC app – it also shows how the number was worked out.
Compare it with the code on your latest payslip. If they differ, ask payroll whether they have applied HMRC’s latest notice.
Provide your P45 or update HMRC with your previous employment details.
If you are still on M1/W1/X after about 35 days, contact HMRC.

See all tax codes or check your take-home with the UK salary calculator.

Cumulative and non-cumulative in one table

Seasonal and shift workers show the difference most clearly. Take someone in England who earns £4,500 a month from April to September and £1,000 a month from October to March – £33,000 for the year.

On cumulative 1257L, payroll keeps track of the year to date. The year’s tax comes to £4,086.00 – exactly the tax due on £33,000. On 1257L M1, every summer month is taxed as if you earned £54,000 a year, so part of it falls into the 40% band. In the winter months, the unused tax-free pay is lost. The M1 total is £4,516.00, an overpayment of £430.00.

Basis	Tax each summer month	Tax each winter month	Total for the year
1257L M1 (non-cumulative)	£752.67	£0.00	£4,516.00
1257L (cumulative)	varies – catches up	refunds unused allowance	£4,086.00

Why HMRC sometimes wants a code to be non-cumulative

HMRC’s PAYE Manual says the week 1 / month 1 basis exists to stop an employer making heavy deductions or paying refunds. When HMRC lowers your code part-way through the year, operating the new code cumulatively could take several months’ extra tax from a single payslip. Month 1 spreads the effect instead. It also stops payroll paying refunds: HMRC’s instructions for D0 and NT on a week 1 / month 1 basis say “do not make any refunds”.

Most code types can be applied either way. HMRC’s manual confirms that suffix codes, K codes, D0, D1 and NT can all be issued on a cumulative or a week 1 / month 1 basis. HMRC’s system defaults D0, D1 and NT to week 1 / month 1.

X and “NONCUM” on payslips

GOV.UK’s examples show how these markers appear: 1257L W1 for weekly pay, S875L M1 for monthly pay and C663L X when pay dates vary. Some payroll software prints “NONCUM” instead. They all mean the same thing: this payday is taxed on its own. Codes with an S or C prefix work the same way, using Scottish or Welsh bands for the period.

When the marker disappears

At 6 April: employers must not carry the W1 or M1 part of a code into the new tax year.
When HMRC sends a cumulative code: payroll applies it and catches up on tax for the year so far. That can mean a refund on the next payday or, if you’ve paid too little, a higher deduction.
Codes issued near the year end: HMRC stops sending codes to employers after 13 March. If an employer gets a code too late to use, GOV.UK tells it to use the code in the new tax year; any difference for the old year is picked up at year end.

W1 with irregular hours: a three-week example

Weekly-paid agency and zero-hours workers see the same effect week by week. Suppose your first two weeks in a new tax year bring no pay and week 3 brings £900. On 1257L W1, week 3 gets only one week’s £241.73 of free pay, so tax is £131.65.

On cumulative 1257L, payroll gives three weeks of free pay (£725.19) against your £900 pay to date, so tax is £34.96. The £96.69 difference comes back only after the code becomes cumulative or the year ends.

Frequently asked questions

Does M1 mean I pay more tax?

Only if your pay varies or you had unused allowance earlier in the year. On steady pay all year, tax is the same as the cumulative version.

What is the difference between W1 and M1?

W1 is used if you're paid weekly, M1 if monthly. Both mean non-cumulative.

What does X mean?

Non-cumulative for people with irregular pay dates – it works the same way.

Is a W1/M1 code ever permanent?

Codes set mid-year for new benefits or pensions can stay non-cumulative until 5 April. HMRC usually issues a normal code for the new tax year.

Why does a non-cumulative code overtax variable pay?

Each month only gets one-twelfth of the allowance and of the £37,700 basic-rate band. Big months spill into 40% and small months waste allowance, and nothing balances out until year end.

Can a K code or D0 be on month 1?

Yes. HMRC can issue suffix codes, K codes, D codes and NT on either basis. D0, D1 and NT default to week 1 / month 1.

Does HMRC send new codes in March?

HMRC’s last date for issuing a code to an employer is 13 March. Changes after that are handled at year end through the P800 process.

Will my P60 show that I was on a non-cumulative code?

The P60 shows the code in use on 5 April, so it may show the M1 or W1 if the code hadn’t changed by then. Your payslips show the code on each payday.

Is a non-cumulative code ever better for me?

Yes, when your code has just been reduced. On a cumulative basis, payroll would collect all the extra tax since 6 April on the next payday. Week 1 / month 1 spreads it instead, which is why HMRC uses it after mid-year reductions.

Sources

Figures are taken from official government publications and were last reviewed on 2026-10-03.