What Is 1257L M1? Emergency Monthly Code Explained

If your payslip shows the tax code 1257L M1, the first part looks completely normal, it is the standard code millions of people are on, but those two extra characters change everything. The M1 suffix tells your employer to forget the past and tax you as though this month is the only month that has ever happened. For someone with an uneven pay history during the year, that quietly costs money until it is fixed.

What the M1 Suffix Actually Switches Off

A normal tax code does quiet, clever maths every payday. It looks at everything you have earned and every pound of tax you have paid since 6 April, works out how much allowance you have accumulated, and adjusts that period's deduction so the running total is always correct. This is called operating on a cumulative basis.

The M1 tax code turns that off. The "M1" stands for Month 1, and it instructs payroll to treat every single month as if it were the first month of the tax year. There is no look-back, no running total, and no self-correction.

This makes 1257L M1 a type of non-cumulative tax code. It is HMRC's safe default when your employer does not yet have the full picture of your year, most often because you started a new job without handing over a P45.

Non-cumulative (Month 1 / Week 1) basis

A way of operating PAYE where each pay period is taxed on its own, using a single period's share of the Personal Allowance and tax bands, with no reference to earlier pay or tax in the year. It prevents large one-off corrections but cannot reclaim earlier overpayments automatically.

Why It Usually Overtaxes You: A 2026/27 Example

The damage from M1 shows up when your earnings in the year so far are uneven, which is exactly the situation when you have just changed jobs.

Take Marcus, who is unemployed for the first three months of 2026/27 (April to June) and then starts a job in July paying £3,500 a month. He is put on 1257L M1.

Under M1, payroll gives him just one month's allowance (£1,047.50) each month and taxes the rest, ignoring the three months of allowance he never used while out of work.

On the correct cumulative code, Marcus has accumulated £4,190 of allowance by July (four months at £1,047.50), which more than covers his £3,500 pay, so he should owe almost nothing that month. On M1 he pays around £490 in tax he does not really owe. The shortfall comes back, but only once a cumulative code is restored, not before.

Note that M1 is not always worse: someone with high, steady pay every month may pay roughly the right amount under either basis. The problem is specifically the lost benefit of allowance built up during low-earning earlier months.

How to Get Back onto a Cumulative Code

The fix is to give HMRC the information it is missing so it can issue a cumulative code:

1. Hand over your P45. Give your new employer the P45 from your previous job. It shows your year-to-date pay and tax, letting payroll switch to a cumulative basis, often within one pay run.
2. Complete the starter checklist. No P45? Fill in HMRC's starter checklist accurately. Your answers (Statement A, B or C) decide the code used while HMRC catches up.
3. Check your live code. Use HMRC's online service to check your tax code and confirm whether a cumulative code has already been issued, rather than waiting to be told.

Once a cumulative 1257L code arrives, payroll recalculates on a year-to-date basis and any tax you overpaid under M1 is refunded automatically through your next payslip. If the tax year happens to end first, HMRC issues a P800 calculation and refunds you directly.

“The M1 on 1257L is small print with a price tag. It is not a penalty, just HMRC's cautious default, and a cumulative code undoes it.”

Related Terms

• M1 Tax Code - the full breakdown of the Month 1 emergency code and how it appears on payslips.
• Non-Cumulative Tax Code - the wider category that 1257L M1 belongs to, covering both Month 1 and Week 1 bases.
• Check My Tax Code - confirm which code is live on your record and whether a cumulative code has replaced the emergency one.

People also ask

Will I get my money back from a 1257L M1 code?

Yes. Once HMRC issues a cumulative code, your employer's payroll recalculates your tax on a year-to-date basis and refunds any overpayment through a future payslip, usually the next one. If the tax year ends before the code is corrected, HMRC sends a P800 tax calculation and refunds you directly.

What is the difference between M1 and W1 on a tax code?

They are the same non-cumulative basis applied to different pay frequencies. M1 (Month 1) is used for monthly-paid employees and gives one-twelfth of the Personal Allowance each month. W1 (Week 1) is used for weekly-paid employees and gives one-fifty-second of the allowance each week. Both ignore earlier pay and tax in the year.

Is 1257L M1 a bad tax code?

It is not a punishment, but it is rarely the right long-term code. Because it ignores your year-to-date position, it often deducts more tax than you actually owe, especially if you had low or no earnings earlier in the year. The aim is always to move onto a cumulative 1257L code, which restores accurate, self-correcting deductions.

Why am I on Month 1 tax when I gave my employer my details?

Even with details supplied, your employer may apply 1257L M1 temporarily until HMRC confirms a cumulative code, particularly if you had no P45 and completed a starter checklist. It is a holding position. Once HMRC processes the information and issues a cumulative code, payroll switches over and corrects any overpayment.