Guide
Why Did My Tax Code Change to M1 or W1?
An M1 or W1 after your tax code changes how payroll calculates tax, even if the number in your code stays the same. Here is how it differs from cumulative tax, why your deduction can change, and what to check with payroll or HMRC.
Quick answer
M1 means month 1 and W1 means week 1: payroll calculates tax using only that pay period, ignoring earlier pay and tax. Cumulative tax uses the tax year so far. M1/W1 can produce too much or too little tax; the suffix alone does not prove an overpayment.
On this page
What M1 and W1 actually mean in plain English
Normally, your employer calculates your tax by looking at everything you have earned since 6 April — your total pay, your total allowance used, and the total tax already deducted. This is called cumulative tax, and it is how most people are taxed most of the time.
When your code has M1 (monthly) or W1 (weekly) after it, payroll stops looking at the year so far. Instead, it calculates tax on just this one pay period, as if it is the only month or week that exists. Some payroll systems show X or NONCUM instead — they all mean the same thing.
GOV.UK calls these emergency tax codes. A changed deduction is a reason to check the details, but emergency treatment does not always mean you have paid too much.
Why the same code number gives a different tax amount
This is the bit that confuses most people. Your code might still say 1257L — the same number and letter as before. But with M1 after it, the calculation changes completely.
On cumulative 1257L, payroll uses the allowance and tax bands available up to the current tax month. It calculates tax on pay to date, then takes account of tax already deducted. This can produce an extra deduction or a refund.
On 1257L M1, payroll uses one month of the allowance and tax bands each time. It ignores earlier pay and tax, so it cannot correct earlier deductions through that calculation. With steady pay and the same code, the two bases can give similar deductions.
Worked example — the same person on both codes
Alex receives £3,000 each month from April to November, then £3,000 plus a £10,000 bonus on 20 December 2026, in tax month 9. Assume all this pay is taxable, with no pension deductions or other adjustments. Earlier pay is 8 × £3,000 = £24,000; December pay is £13,000; pay to date is £37,000.
| Input | 1257L cumulative | 1257L M1 |
|---|---|---|
| Pay used in the calculation | £37,000 to date | £13,000 this month |
| Allowance and tax bands | Available through month 9 | One month only |
| Tax already deducted | Taken into account | Ignored |
M1 can put more of a bonus into higher tax bands in that month because it only uses one month of bands. Cumulative PAYE also includes the whole bonus when paid; it does not schedule the bonus tax over future months. Later cumulative calculations may change the overall position.
This compares the calculation inputs, not exact deductions or a promised refund. Payroll needs the applicable code, pay date and earlier tax deducted to establish the cumulative deduction. Our calculator and checker use annualised estimates and do not reconstruct that history.
If one payslip looks isolated from the rest of the year
These next pages help you decide whether the issue is emergency treatment, a wrong code, or just a short-lived setup problem.
Common reasons HMRC switches you to M1 or W1
Missing information after starting a job. HMRC may not yet have the details of your earlier income. Give payroll your P45 or, if you do not have one, the information requested in a starter checklist. Missing a P45 does not mean every starter must receive M1/W1: the starting code depends on the information available.
Changes in taxable income or benefits. GOV.UK also lists starting work after self-employment and receiving company benefits or the State Pension as possible reasons for emergency tax. Check your coding notice or HMRC account for the reason in your case.
A bonus can change the amount deducted without any code change. Its arrival alone does not prove that HMRC switched you to M1/W1.
How long it usually lasts
For a new job, HMRC says updating the code can take up to 35 days from starting. If it is still wrong after that, check and update your details with HMRC. This is not a guarantee that every emergency code will end within 35 days.
Some emergency codes, including those used after starting company benefits or the State Pension, can remain until the end of the tax year. The reason for the code matters more than a blanket waiting period.
How to speed up the fix
Give payroll your P45 or starter details. The starter checklist is information for your employer to use when setting up payroll; it is not a form you send directly to HMRC.
Check your HMRC personal tax account. Review your jobs, pensions, benefits and estimated taxable income. Use HMRC’s tax-code service to correct missing or outdated details.
Check when payroll received any replacement code. Once HMRC changes a code, it should normally appear on the next or following monthly payslip, or the third weekly payslip. If it has not appeared, ask your employer. Follow HMRC’s contact guidance if you cannot resolve the information online.
What happens to the overpaid tax
If HMRC has the necessary income details and confirms you have overpaid in the current tax year, it can ask your employer to refund you when the new code is used. This is usually the next or following monthly payday, or the third weekly payday. A cumulative calculation can also collect tax owed; a refund is not automatic just because M1/W1 disappears.
After the tax year, HMRC checks whether the right tax was paid and writes if there is a difference. If you receive a P800 showing a refund, follow its instructions. Not every emergency code results in a P800 or a refund. See HMRC refund instructions.
If your pay varies — commission and bonus earners
If you earn commission, overtime or bonuses, a month with higher pay can use up its share of the lower tax bands. On M1/W1, unused bands from earlier periods are not brought into that calculation. Check the basis as well as the code number before comparing deductions.
If this sounds like your situation, read our detailed guides on how commission interacts with the tax system:
The examples illustrate tax effects under stated assumptions; they are not a substitute for payroll checking your pay and tax to date.
What to check
- Look at the end of your tax code — M1, W1, X, or NONCUM means you are on non-cumulative (emergency) basis.
- Compare the code, suffix and taxable pay with your previous payslip; more than one of these can change the deduction.
- Check whether payroll has your P45 or starter details and whether HMRC has the right employment and income information.
What to do next
- Ask payroll to check the code basis using your pay and tax to date. Our checker gives an estimate, not a cumulative payroll calculation.
- If you have a P45 from a previous employer, give it to payroll; complete a starter checklist if you do not.
- Log in to your HMRC personal tax account at gov.uk to check what code they have on file.
Try the tool
Check your payslip or model a change.
Use the checker if you already have a payslip. Use the calculator if you want to model take-home pay or salary-sacrifice changes before payday.
About this guide
Published by IsMyPayRight to help you understand pay and deductions. Guides use official reference material and practical examples, with AI assistance in content preparation.
This is general guidance, not a professional review of your circumstances. Read our editorial process and corrections information.
Methodology and sources
See how the calculations work, which sources they use, how results are tested, and which payroll details the estimates cannot verify.
Common questions
- What is the simplest difference between cumulative and M1/W1?
- Cumulative looks at the tax year so far. M1 or W1 looks only at the current pay period.
- Can M1 or W1 stop an automatic refund on the next payslip?
- Yes. Because it does not look back across earlier periods in the same way, it can stop the normal catch-up effect you see under cumulative PAYE.
- Does a new tax year remove M1 or W1 automatically?
- P9X says employers should not carry week 1 or month 1 markings over when copying a code into the new tax year.
Official sources
Official sources
Use these references to check the rules behind this guide. Check the tax year and your circumstances before applying an example to your own pay.