What does M1 on a tax code mean?

Per gov.uk, M1 (sometimes shown as W1 for weekly payrolls, or X) marks a tax code as "non-cumulative". Each pay period is taxed in isolation — your employer ignores the year-to-date pay and tax position. It is typically attached to 1257L (so "1257L M1") and used as an emergency basis after a job change or when HMRC has incomplete information.

You usually go onto M1 after starting a new job before HMRC has issued a firm cumulative code, after a P45 has been processed showing unusual year-to-date figures, or when HMRC needs to neutralise a possible over- or under-deduction. It is meant to be temporary — most M1 codes are replaced by a cumulative code within a pay period or two.

"Emergency tax" is a loose term that usually refers to a 1257L W1/M1 code. It is not punitive on its own — you still get the personal allowance — but because each pay period is taxed in isolation, you may pay slightly too much (or too little) tax in early pay periods. HMRC reconciles when the cumulative code arrives or at year-end through a P800.

How much extra tax does M1 take?

On standard 1257L M1 the monthly allowance is £12,570 / 12 = £1,047.50. If you change jobs mid-year and have unused allowance from earlier months, M1 cannot back-claim it — so you pay tax on more income than necessary until cumulative takes over. The difference usually refunds in-year once HMRC issues a cumulative code.

How do I move from M1 to cumulative?

Give your employer your P45 from the previous job, or complete the starter checklist accurately. Sign in to the HMRC personal tax account and confirm employments. HMRC will normally re-issue a cumulative code within a few days. Once payroll receives it, the next payslip evens up the year-to-date PAYE.

Educational guidance only — not regulated tax, financial or payroll advice. Verify with HMRC or your payroll team before acting.

UK · Tax codes · M1

Tax code M1 — emergency Month-1, non-cumulative basis

If your payslip shows a tax code ending in M1 (for monthly payroll) or W1 (weekly) or simply X, the code is being applied on a non-cumulative basis. Each pay period is taxed in isolation — year-to-date pay and tax are ignored. It is the standard emergency basis after a job change, when HMRC has incomplete information about your year so far.

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What this code means

Per gov.uk's tax-code guidance, a normal cumulative code (like 1257L without any marker) tells payroll to look at your year-to-date taxable pay and year-to-date PAYE on every payday and balance accordingly — refunding overpaid tax in earlier periods if appropriate. M1 (Month 1) and W1 (Week 1) switch this off. Each pay period stands alone. Your employer applies 1/12th (monthly) or 1/52nd (weekly) of the year's allowance and bands to that period only.

The result on 1257L M1 is that you get £1,047.50 of allowance per month, no matter what has happened earlier in the tax year. If you have unused allowance from previous months (for example after a gap of unemployment), M1 cannot claw it back — you wait for the cumulative code, or for the year-end P800 reconciliation, to recover the over-deduction. M1 is therefore not a different tax rate, it is a different accounting method.

M1 = Month-1, W1 = Week-1, X = generic non-cumulative marker. Same underlying mechanic.
Allowance still applies. 1257L M1 still gives you the £12,570 allowance — just sliced evenly.
Designed to be temporary. HMRC normally re-issues a cumulative code within one or two pay runs.

Who gets M1 / W1 / X

Per HMRC, the M1 marker is added when full year-to-date information is not available, or when applying year-to-date figures would risk a wrong refund or deduction. The most common triggers are below.

Starting a new job without a P45: Per gov.uk's starter-checklist guidance, if you start a new job without a P45 and indicate this is your only or main job, HMRC normally applies 1257L M1 (or W1 for weekly payrolls) until they can confirm a cumulative code. Each pay period is then taxed in isolation, ignoring the year-to-date.
After a job change with irregular earnings on the P45: If your P45 shows unusual year-to-date pay or tax — for example a large redundancy payment or a long gap with no income — HMRC may issue an M1 code to your new employer while it reconciles, then switch to cumulative once the picture is clear.
After HMRC corrects a previous coding error: When HMRC unwinds a duplicate employment, fixes an over-issued allowance, or removes a benefit in kind, it may attach M1 temporarily so payroll cannot retrospectively over- or under-deduct. The cumulative version follows once year-to-date figures are re-set.
Pension drawdown — first payment: Pension providers commonly apply M1 (often on 0T or 1257L) to a first drawdown payment because they have no record of year-to-date pay from other sources. HMRC then issues a cumulative code from the second payment onwards.
Coding notice delays at the start of a tax year: At the start of a new tax year, if HMRC has not yet issued the firm code, payroll may default to a Month-1 / Week-1 version of last year's code or 1257L until the new coding notice arrives.

Worked example — £30,000 job started in October on M1

New job in month 7 (October), £2,500 / month gross. No earlier income in the tax year. Compare 1257L cumulative versus 1257L M1.

Step (first pay run, October)	1257L cumulative	1257L M1
Gross pay this month	£2,500	£2,500
Year-to-date allowance available	£7,332.50 (7 × £1,047.50)	£1,047.50 (1 month only)
Taxable pay this month	£0 (sheltered)	£1,452.50
PAYE this month	£0	£290.50
Difference	M1 over-deducts ≈£290 in month 1

That over-deduction reverses once HMRC issues a cumulative 1257L — typically the next pay run produces a refund through payroll. If the cumulative code does not arrive by year-end, a P800 reconciliation in the autumn refunds the difference.

Common mistakes & red flags

M1 stuck for months. If you are still on M1 after two or three pay runs and HMRC has your P45, push payroll and HMRC for a cumulative code.
Large lump-sum on M1. A bonus or back-pay on a non-cumulative code can push the period into higher-rate territory even though the annual income wouldn't. Cumulative corrects this.
M1 paired with 0T. Means no allowance applied per-period. Provide a P45 / starter checklist urgently.
Multiple W1/M1 codes across jobs. Suggests HMRC has not picked which is your main employment. Confirm in the personal tax account.

What to ask payroll

“Why was M1 (or W1 / X) applied?”
“Have you received my P45 and is it processed?”
“What HMRC coding notice are you working from?”
“If HMRC issues a cumulative code, will the in-year refund process automatically?”
“Can you confirm year-to-date pay and PAYE on my record?”

How to fix M1 / W1 / X

Hand over your P45 or complete the starter checklist accurately. Tick “only or main job” if true.
Sign in to the HMRC personal tax account. Confirm employments and update the income estimate at gov.uk.
Wait for the cumulative code. HMRC normally re-issues within a few days. Payroll applies it from the next pay run.
Refund automatic. Once cumulative kicks in, any over-paid PAYE refunds through payroll within the same tax year.

Frequently asked questions

What does M1 on a tax code mean?: Per gov.uk, M1 (sometimes shown as W1 for weekly payrolls, or X) marks a tax code as "non-cumulative". Each pay period is taxed in isolation — your employer ignores the year-to-date pay and tax position. It is typically attached to 1257L (so "1257L M1") and used as an emergency basis after a job change or when HMRC has incomplete information.
Why am I on M1?: You usually go onto M1 after starting a new job before HMRC has issued a firm cumulative code, after a P45 has been processed showing unusual year-to-date figures, or when HMRC needs to neutralise a possible over- or under-deduction. It is meant to be temporary — most M1 codes are replaced by a cumulative code within a pay period or two.
Is M1 emergency tax?: "Emergency tax" is a loose term that usually refers to a 1257L W1/M1 code. It is not punitive on its own — you still get the personal allowance — but because each pay period is taxed in isolation, you may pay slightly too much (or too little) tax in early pay periods. HMRC reconciles when the cumulative code arrives or at year-end through a P800.
How much extra tax does M1 take?: On standard 1257L M1 the monthly allowance is £12,570 / 12 = £1,047.50. If you change jobs mid-year and have unused allowance from earlier months, M1 cannot back-claim it — so you pay tax on more income than necessary until cumulative takes over. The difference usually refunds in-year once HMRC issues a cumulative code.
How do I move from M1 to cumulative?: Give your employer your P45 from the previous job, or complete the starter checklist accurately. Sign in to the HMRC personal tax account and confirm employments. HMRC will normally re-issue a cumulative code within a few days. Once payroll receives it, the next payslip evens up the year-to-date PAYE.

Tax code 1257L — standard cumulative allowance
The code M1 is normally attached to.
Tax code 0T — no allowance, full bands
Often appears with W1/M1 after a job change with no P45.
Tax code BR — flat 20%
Related “emergency” flavour for second jobs.
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Last reviewed

Last reviewed: 16 May 2026

Next review: 6 April 2027

Reviewed each tax year and sooner if HMRC changes emergency-tax rules.

Educational guidance only — not regulated tax, financial or payroll advice. Always verify with HMRC or your payroll team before acting. To run a real check, the wrong-tax-code checker is the place to begin.