Month 1 and Week 1 tax codes explained

A tax code ending in M1 (Month 1) or W1 (Week 1) tells payroll to work out your tax on a non-cumulative basis. Each payday is taxed as if it were the first of the tax year: you get one month’s or one week’s share of your allowance and tax bands, and nothing for earlier in the year. On 1257L M1 that means £1,048 of tax-free pay each month and no catch-up for months you did not work or earned less. The marker X does the same thing for payrolls that cannot show W1 or M1.

Cumulative and non-cumulative compared

Normal PAYE is cumulative. Payroll adds up your pay since 6 April, compares it with the allowance and bands due to date, and works out the tax due to date. The tax taken this payday is that figure minus what you have already paid. Unused allowance carries forward, and overpaid tax is refunded automatically through payroll.

Non-cumulative PAYE drops the year-to-date comparison. Each period is self-contained, so:

• unused allowance from earlier months is lost to payroll (though not to you; it comes back after the year ends)
• a low-paid month does not trigger a refund of earlier tax
• a high-paid month is taxed against that month’s band only, so the higher rate can kick in sooner.

A worked example

Say you start an NHS job in October on £27,600 a year (£2,300 a month), having had no taxable income since 6 April, and you live in England.

	1257L (cumulative)	1257L M1 (non-cumulative)
Tax each month, October to February	£0	about £250
Tax in March	about £246	about £250
Tax for the year	about £246	about £1,502

On the cumulative code, your pay to date stays under the allowance to date until March, so almost no tax is taken. On M1, each £2,300 month is taxed at 20 per cent on everything above £1,048, which is about £250 a month for six months. The difference, around £1,256, has been overpaid, and you have to wait for it to come back. The take-home pay calculator shows what a normal cumulative month should look like for your salary.

For someone who has worked steadily since April, M1 makes very little difference, because their allowance to date has already been used at roughly the same pace.

When HMRC or payroll uses it

The most common reason is starting a new job without a P45, or before HMRC has matched the job to your record. Payroll uses the emergency code 1257L M1 (or W1, or X) until HMRC sends a proper one. The emergency tax codes guide covers that situation step by step.

HMRC also issues non-cumulative codes on purpose. If your code changes part way through the year, a cumulative code would make payroll recalculate everything since April and either refund or collect a lump in one go. HMRC sometimes adds M1 or W1 to the new code so that the change only affects pay from now on, and any difference for the earlier months is settled after the year ends. You may see this after a change to a benefit in kind, a second job starting or stopping, or a correction to estimated income.

The marker can go on any code. You might see 1257L M1, S1257L W1, C1100L M1 or 0T M1.

When it can leave you paying too little

Non-cumulative coding usually means too much tax, but it can work the other way. If you earned a lot earlier in the tax year (from a previous job, or a large bonus) and then move to a new job on an M1 code, the new job gives you a full month’s allowance every month as if you had used none. By the end of the year you may have had more tax-free pay than £12,570, and HMRC will collect the underpayment, usually by reducing your code the following year. Underpaid tax and tax code adjustments explains how that works.

How to get it removed

1. Give payroll your P45 from the previous job if you have one, or complete the starter checklist accurately.
2. Check your HMRC personal tax account or the HMRC app. The job should show with its code and an estimate of your income for the year. Correct the estimate if it is wrong.
3. If the M1 or W1 is still there after two or three paydays, contact HMRC and ask whether the code can be made cumulative. Have your National Insurance number and your employer’s PAYE reference ready.

When a cumulative code reaches payroll, the next payday recalculates the whole year to date and refunds any overpayment in that pay. If the year ends before that happens, HMRC reconciles your record after 5 April and tells you what is due; claiming a tax refund covers the routes if you need to chase it.