1257L M1 and W1: What 'Month 1' Means on Your Payslip

1257L M1 means you get the standard Personal Allowance of £12,570, but your employer must tax each pay period in isolation — roughly £1,047.50 of tax-free pay this month, with no account taken of what you earned or paid in tax earlier in the tax year. 1257L W1 is the identical rule for weekly paid staff (about £241.73 of free pay per week). It is a "non-cumulative" code, and it is usually a holding position after a job change rather than a permanent setting.

• M1 = month 1, W1 = week 1. Some payslips show the same thing as "X" or "non-cum".
• The tax code number is unchanged: 1257L still gives the full £12,570 allowance, just sliced evenly rather than accumulated.
• If you had a gap in earnings this tax year, a month 1 basis usually means you overpay until it is corrected.
• If you have two jobs, or a duplicated allowance, it can mean you underpay and owe tax later.
• Only HMRC can switch the code back to cumulative — your employer cannot remove the M1/W1 marker on request.

What M1 and W1 actually mean on a payslip

A normal (cumulative) tax code works on a running total. Each payday your employer looks at your total pay for the tax year so far, the total tax-free allowance you have built up so far, works out the tax due to date, and deducts the difference from what you have already paid. That self-corrects: if you paid too much in July, August's payslip hands some back.

A non-cumulative code switches that off. With 1257L M1, month 7 is treated as though it were month 1: one twelfth of the allowance, one twelfth of the basic-rate band, and no memory of anything before it.

The same slicing applies to the rate bands. For England, Wales and Northern Ireland, the £37,700 basic-rate band becomes roughly £3,141.67 a month, and 40% tax starts once monthly pay passes about £10,428 (a twelfth of £125,140). HMRC's payroll tables round free pay to the penny in their own way, so the figure on your payslip may differ from £1,047.50 by a few pence.

Scotland

A Scottish code looks like S1257L M1. The mechanics are identical — the difference is that Scottish income tax rates from 19% to 48% are applied to your non-savings income, and those bands are sliced into monthly or weekly chunks in the same way. Savings and dividend income follow the UK rates wherever you live.

Why it appears after a job change

The month 1 basis exists to stop a new employer giving you a big chunk of unused allowance, or a big refund, based on information they cannot verify. The common triggers are:

• No P45 handed to the new employer. Without the pay and tax figures from your last job, payroll has nothing to accumulate from, so HMRC's starter rules put you on 1257L W1/M1.
• The P45 is from an earlier tax year. A P45 dated before 6 April is no use for a running total in the current year.
• The starter checklist answer. Declaring that this is your only job and you have had no other taxable income or benefits since 6 April normally produces a cumulative 1257L. Saying you have had another job or taxable benefit since 6 April normally produces 1257L on a week 1/month 1 basis instead.
• A mid-year code change. If HMRC increases your allowance part-way through the year — for example after removing a company benefit or a wrongly estimated figure — it sometimes issues the new code on a month 1 basis so you do not receive a large one-off refund through payroll before the facts are confirmed.
• A first flexible pension withdrawal. Pension providers commonly apply 1257L on a month 1 basis to a first taxable lump sum, which is why those payments are so often overtaxed.

Because it is the code HMRC specifies for new starters with incomplete information, 1257L W1/M1 is what most people mean when they talk about being put on emergency tax. Our guide to emergency tax codes covers the other codes you might see, including BR, 0T and D0.

Does a month 1 basis cost you money?

Not always. If you worked steadily all year and simply moved from one job to another with no gap and no overlap, 1257L M1 gives you roughly the right allowance each month and the outcome can be close to correct. The problems come from gaps and overlaps.

Example 1: a gap in earnings, so you overpay

Ravi earns nothing from April to July, then starts a job on 1 August (tax month 5) at £3,000 a month. His code is 1257L M1.

• On M1: each month, £3,000 − £1,047.50 = £1,952.50 taxable at 20% = £390.50 a month. Over eight months, £3,124.
• Correct position for the year: total pay £24,000, less the £12,570 allowance = £11,430 taxable at 20% = £2,286.
• Overpaid: £838 — which is exactly the four months of allowance he never got credit for (4 × £1,047.50 × 20%).

On the cumulative code the first two payslips are tax-free because the allowance built up from April has not been used. That is the money the month 1 basis holds back.

Example 2: two jobs, so you underpay

Chloe's main job pays £25,000 and already uses her whole Personal Allowance. She takes a weekend job paying £1,000 a month, and HMRC mistakenly issues 1257L M1 to it. Because £1,000 is below the monthly free pay of £1,047.50, the second job deducts no tax at all — but every pound of it should be taxed at 20%, so £200 a month is going unpaid. After six months that is £1,200 HMRC will want back, usually through next year's tax code.

The lesson is that a non-cumulative code is not automatically in your favour or against you. It simply stops the payroll self-correcting, so any error stays in place until someone spots it. You can model both directions with the emergency tax calculator.

What it does not affect

National Insurance is worked out separately for each pay period as standard, so a W1/M1 marker makes no difference to it: employee Class 1 NI is 8% between £12,570 and £50,270 a year and 2% above, whatever your tax code. Student loan deductions are also per-period and unaffected.

Getting back onto a cumulative code

Your employer must operate whatever code HMRC tells them to operate. They cannot drop the M1 or W1 suffix because you have asked them to, so complaining to payroll rarely works. These are the routes that do:

1. Hand over your P45 from the current tax year. If you still have parts 2 and 3 and the leaving date falls on or after 6 April, give them to your new employer. That supplies the pay and tax to date the payroll needs.
2. Complete the starter checklist accurately. If the only reason you are on a month 1 basis is a hurried tick-box answer, tell payroll and ask them to submit the correct statement. The form is on GOV.UK at gov.uk/guidance/starter-checklist-for-paye.
3. Tell HMRC directly. Through your Personal Tax Account you can check and update the jobs and pensions HMRC thinks you have, and remove any employment that has ended. Start at gov.uk/check-income-tax-current-year. HMRC then issues a revised code to your employer, which usually takes effect on the next full payroll run.
4. Wait for the new tax year. At the start of a tax year, employers carry the number forward but drop the week 1/month 1 marker, so an uncorrected 1257L M1 becomes a cumulative 1257L from 6 April. That fixes the going-forward position but does not refund the earlier year by itself.

If a cumulative code arrives before the end of the tax year, any overpayment comes back through your payslip — the running total recalculates and the refund shows up as a smaller deduction, or occasionally as a negative tax figure. If the year ends while you are still on a month 1 basis, HMRC reconciles the year automatically from the payroll data it holds and sends a P800 calculation with a refund if one is due. That process runs over the months after the tax year ends rather than immediately.

Before you contact anyone, it is worth checking whether the number itself is right as well as the basis. A code can be non-cumulative and wrong — for example 1185L M1 because of an out-of-date benefit figure. The tax code checker explains what each part of your code is doing.

What to do next

Work out the size of the problem before you act. Put your pay, your code and the month you started into the emergency tax calculator to see what a month 1 basis is costing you against a cumulative code, then use the tax code checker to confirm the allowance in your code is the one you are entitled to. If the code is wrong as well as non-cumulative, updating your employment details with HMRC fixes both at once.