M1 and W1 Tax Codes: What They Mean and What to Do

An M1 or W1 tax code is an emergency tax code. It tells your employer to work out your tax on that pay period alone, ignoring what you’ve already earned this year. M1 applies to monthly pay and W1 to weekly pay. GOV.UK warns the result can be too much tax deducted, or too little.

The cause sits in what HMRC knows about you, not in anything you did wrong. An emergency code is a placeholder, applied when HMRC doesn’t yet have a full picture of your income and swapped for a proper code once it does.

Two letters at the end of a code are easy to skim past. They sit in the same small print as your National Insurance number, and nothing about them shouts for attention.

What makes them worth spotting is the money. A standard code spreads your Personal Allowance across the year and keeps a running total of what you have earned and paid. An emergency code throws that running total away every payday, and that is where overpayments start.

What an M1 W1 tax code means

Your tax code tells your employer how much tax-free income to give you before deducting tax. Add an M1 or W1 to the end and the calculation changes character completely.

Two questions come up constantly: what does M1 mean on tax code lines, and what does W1 mean on tax code lines? Both answers come down to pay frequency.

M1 is the monthly pay tax code suffix, W1 the weekly pay tax code suffix, and neither changes the number in front of it.

For 2026/27 the GOV.UK emergency tax codes list attaches those suffixes to the standard 1257L code:

• A 1257L W1 tax code is used when you’re paid weekly, giving an emergency tax code W1 on your payslip.
• A 1257L M1 tax code is used when you’re paid monthly, giving an emergency tax code M1 instead.
• A tax code ending in X is used when your pay dates vary.

Some payroll software prints NONCUM rather than a suffix, according to GOV.UK, meaning the same thing. Where the tax code on payslip records finishes with anything other than those markers, HMRC emergency tax rules aren’t being applied to you at all.

The formal term is a non cumulative tax code. Cumulative means each payday looks back across the whole year. Non-cumulative means each payday stands alone, which is why the W1 M1 tax code meaning is also described as a week 1 month 1 tax code.

How much tax an emergency tax code takes

This is the bit most guides skip, and it explains the shortfall better than anything else on the payslip.

Your Personal Allowance is not handed over in one lump each April.

For example HMRC’s rates and thresholds for employers 2026 to 2027 put it at £12,570 a year, which payroll splits into £1,048 a month or £242 a week. A cumulative code lets unused slices carry forward. An emergency code doesn’t.

Out of work for two months? A cumulative code would hand you three months of allowance on your first payday back. An emergency code gives you one month and leaves the rest out of reach.

So how much tax do you pay on an emergency tax code? Here is £3,000 in a month on 1257L M1 for 2026/27:

• Monthly Personal Allowance under the code: £1,048.
• Taxable pay for the month: £1,952.
• Tax at the 20% basic rate: £390.40.
• The same £3,000 on a cumulative code, in month three with no earlier pay, would attract no tax at all.

That gap isn’t a penalty and it isn’t permanent. It’s unused allowance sitting somewhere you can’t reach until the code is corrected. National Insurance is worked out separately and the suffix doesn’t touch it.

Why is my tax code W1 or M1?

An emergency code answers missing information rather than passing judgement on you. GOV.UK points to a change in circumstances where HMRC has not received income details in time.

Triggers include:

• Starting work without a P45, the P45 new job tax code trigger.
• Returning from a career break or a first job after study.
• Returning to employment after a spell of self-employment.
• Company benefits starting, or a tax code state pension change.
• Taking a taxable lump sum from a pension for the first time.
• Starting or leaving work part-way through the tax year.

A tax code second job situation is worth separating out, because it usually draws a BR code rather than an emergency one.

Your allowance is already sitting with your main job, so the second employer taxes from the first pound. HMRC’s PAYE manual lists 0T and D0 on secondary sources too, so which one you get turns on your total income. Different code, different reason, similar sting on the payslip.

Codes that look like emergency codes

Some of the worst payslip surprises come from tax codes that are not emergency codes at all. Telling them apart saves chasing the wrong fix, and it is worth two minutes of your time.

• BR taxes every pound from that employment at the basic rate with no allowance applied.
• 0T withholds the allowance too, but tax climbs through the bands as pay rises.
• A code starting with K works in reverse, adding to taxable pay rather than reducing it.

None of those three carry a suffix. Only W1, M1, X or NONCUM on the end makes a code an emergency one, so a code reading BR or 0T alone is doing exactly what HMRC intended.

How long does an emergency tax code last

An M1 or W1 tax code has two possible lifespans, depending on what triggered it.

After a new job, GOV.UK states HMRC will usually update the code once both your current and former employer have reported, allowing up to 35 days from your start date. Past 35 days with no change, that is the point to act rather than wait.

Where company benefits or the State Pension caused it, GOV.UK guidance runs the other way: the code stays until the end of the tax year, then reverts in the new one. Same suffix, very different timeline.

Is M1 tax code temporary, then? Yes, though not on a fixed clock. GOV.UK adds that if the change in circumstances left you underpaid, you keep the emergency code until the right tax for the year has been paid, with a tax code underpayment adjustment collecting the difference.

How to get off emergency tax

Clearing a wrong tax code is a matter of getting HMRC what it lacks, and there are three routes:

• Hand your new employer the P45 from your last job, the fastest option where you have one.
• Complete a starter checklist if no P45 exists, since a starter checklist tax code update gives your employer what they must report.
• Update your details yourself in the personal tax account tax code section.

To check tax code HMRC currently holds, the HMRC Income Tax checker shows the code and the estimate behind it. That’s the quickest way to see whether a new code has been issued. On how to change tax code details by phone, the HMRC contact number tax code queries go to is 0300 200 3300.

Your employer can’t change a code on request. They apply what HMRC issues, so the instruction has to come from HMRC.

Claim back emergency tax on an M1 W1 tax code

Overpaid emergency tax from an M1 or W1 tax code doesn’t vanish, and there’s frequently nothing to fill in.

When HMRC issues a corrected cumulative code mid-year, GOV.UK explains your employer works out the difference and refunds it through your pay, producing an in-year tax refund. On monthly pay that lands on the next payslip or the one after; on weekly pay, GOV.UK says the third.

If the code isn’t corrected before 5 April, reconciliation happens after the tax year closes. HMRC checks the figures against employer reports and writes to you where the amount is wrong, setting out how any emergency tax refund gets paid.

Pension withdrawals differ. Where an emergency code hit a lump sum, HMRC has forms to reclaim in-year rather than waiting: the P55, P50Z and P53Z, depending on whether the pot was emptied and what other income exists.

A tax rebate calculator gives a rough estimate of what an overpayment might be worth across several tax years, though only HMRC’s figures settle it.

Before you claim

An M1 or W1 tax code is a signal, not a bill. It’s telling you HMRC is working from incomplete information and rationing your Personal Allowance a payday at a time.

Check the suffix on your payslip, get a P45 or starter checklist to your employer, and confirm what HMRC holds. Correct the code and the money follows, either through payroll or after the tax year closes.

To see what HMRC has on file, the tax code checker guide is a good place to start.

Key takeaways

The short version, if you’re looking at a payslip right now:

• An M1 or W1 tax code carries a suffix that taxes each pay period in isolation.
• For 2026/27 the emergency codes are 1257L W1, 1257L M1 and 1257L X, and some payslips show NONCUM instead.
• The code gives one period’s slice of Personal Allowance, £1,048 a month or £242 a week, with no carry-forward.
• After a new job HMRC usually updates within 35 days, but a State Pension or company benefit trigger runs to the tax year end.
• A P45, a starter checklist or an update through the personal tax account are the three ways to clear it.
• Overpaid tax returns through payroll once the code changes, or after 5 April if it does not.

Common M1 and W1 tax code questions

The questions below cover ground the sections above do not, and each answer draws on published HMRC guidance.

Why has my tax code changed to M1 partway through the year?

A mid-year switch tends to mean HMRC has received information that changes what it expects you to earn, but not enough to run a cumulative calculation safely. Picking up a second source of income does this.

Moving you to a non-cumulative basis stops those new figures being applied backwards to pay you already received, which could otherwise produce a large one-off deduction. It’s protective as much as anything.

The code should revert once HMRC has reconciled the full year’s income, either through an updated code or at the tax year end.

Do you get emergency tax back automatically?

In the ordinary case, yes. GOV.UK describes HMRC checking your figures once it has your income details and asking your employer to refund the difference through payroll, without you filling anything in.

The exception is where HMRC never receives the missing details. Nothing reconciles until it does, which is why supplying a P45 or starter checklist matters more than waiting patiently.

Pension lump sums sit outside this pattern. There the P55, P50Z and P53Z exist precisely because waiting for automatic reconciliation can mean waiting until after 5 April.

Can an emergency tax code make me underpay rather than overpay?

Yes, and it’s the scenario people rarely plan for. With more than one income source each taxed on its own pay period, the allowance can end up applied more than once across the year.

GOV.UK addresses this directly: where you haven’t paid enough, HMRC estimates what is owed and adjusts your code to collect it, potentially spread over more than one tax year.

You also keep the emergency code until the correct tax for that year has been paid, which can hold the suffix in place longer than expected.

What happens to my emergency tax code on 6 April?

The M1 or W1 part doesn’t carry into the new tax year. HMRC guidance for employers is explicit that the suffix gets dropped when codes roll over, so the year starts on a cumulative basis.

That doesn’t make the underlying number right. Where the code still fails to reflect your circumstances, the same checks apply as at any other point in the year.

Any overpayment from the closed year is handled separately, through HMRC’s end-of-year reconciliation rather than the new code.

Do Scottish and Welsh taxpayers get different emergency codes?

The suffix behaves identically, but the code carries a prefix. GOV.UK’s own examples include S875L M1 for a Scottish taxpayer and C663L X for a Welsh one.

The S prefix matters more than the C, since Scottish income tax runs on six bands rather than three for 2026/27. The rate applied to emergency-taxed pay differs accordingly.

Personal Allowance is identical across all three at £12,570 for 2026/27, according to HMRC’s published employer rates.