Changed jobs this summer and spotted an M1 W1 tax code on your payslip? Here's what it means, why it overtaxes you, and how to claim the refund in 2026/27.
Summer is peak job-changing season. New contracts start in August, students move into full-time work, and people switch employers before the autumn. If you have just started somewhere new and your first payslip looks lighter than you expected, check the tax code. If it ends in M1 or W1, that is why.
An M1 W1 tax code is not a mistake exactly, but it usually means you are paying more tax than you should for a month or two. Here is what an M1 W1 tax code means, why a job change sets it off, and how to claim the refund back in the 2026/27 tax year.
Your tax is being worked out on this pay period alone, ignoring everything you earned and paid earlier in the year, because payroll does not yet have your full history.
M1 is the monthly version, W1 the weekly one. Both are what people mean by an emergency tax code. You might see them written as 1257 M1, 1257L W1, or with the letters after your normal code. The number still gives you your personal allowance, but the M1 or W1 flag changes how that allowance is applied.
A normal cumulative tax code looks at your whole year. Each payday it adds up your total pay so far, works out how much tax you should have paid on that running total, subtracts what you have already paid, and charges the difference. That is what smooths everything out and gives you refunds automatically when your income dips.
An M1 or W1 code throws that memory away. It treats every payday as if it is the first one of the year, giving you exactly one month’s or one week’s slice of your personal allowance and nothing else.
Here is where it bites. Say you left a job in May, took June and July off, and started fresh in August. Under a normal code, payroll would know you have several months of unused personal allowance banked, and your first new payslip would be very lightly taxed. Under M1, it ignores all of that. You only get August’s slice of the allowance, so you are taxed as though you have been earning steadily all year when you have not. That unused allowance is real money, and the emergency code quietly parks it.
You start a new job on 1 August 2026 earning £30,000, on tax code 1257L M1.
That looks fine on its own. But you had no PAYE income in June or July, which means two months of personal allowance, about £2,095, went completely unused. On a cumulative code, that unused allowance would have wiped out most of August’s tax bill. On M1 it does not, so you overpay by roughly £400 this month compared with what you actually owe for the year.
Multiply that across a couple of pay cycles and the emergency code can hold onto several hundred pounds of your money until it gets corrected.
It is worth saying that an M1 W1 tax code does not lose you the money for good. Everything the code overtaxes now comes back once your record is complete, either through your payslip or after the tax year ends. The only real cost is the delay, and how long your own cash sits with HMRC instead of in your account. That is exactly why it is worth spotting the code early rather than noticing it in March.
When you leave a job you get a P45, which records your pay and tax for the year so far. When you start the new one, handing over that P45 lets your new employer carry your figures forward and put you straight onto a cumulative code.
An M1 W1 tax code appears when that handover does not happen cleanly. Common reasons: you did not have your P45 yet, you started before the old employer issued it, you filled in the new starter checklist in a way that flagged you as having other income, or there was simply a lag in HMRC linking the two jobs. None of these are your fault, and all of them are fixable.
One thing that catches people out: if the new starter checklist asks whether this is your only job and you tick the wrong box, payroll may assume you have a second income and hand you an M1 W1 tax code or even a BR code by default. If that has happened, correcting the checklist answer with your employer, or updating your employments in the HMRC app, clears it.
The good news is you rarely have to do much. Once your record is complete, the money comes back on its own.
First, give your new employer your P45 if you have not already. Parts 2 and 3 are the ones they need. This is the single fastest fix, because it lets payroll switch you to a cumulative code and refund the overpayment on your next payslip.
Second, check the HMRC app or your Personal Tax Account. Log in with your Government Gateway ID and look at your current employment and tax code. If HMRC has your new job listed with the right start date, the correction is usually already on its way. If your old job still shows as open when you have left it, that is often what is holding things up, and you can update it there.
Third, wait one or two pay cycles. When the cumulative code arrives, payroll recalculates your year to date and any overpaid tax lands as a lump in that month’s take-home. You do not need to fill in a form for this. It is automatic once the code is right.
If the tax year ends and you are still owed money, HMRC reconciles everyone’s PAYE after 5 April and sends a refund by cheque or bank transfer, usually with a P800 letter explaining it.
If you are three months into the new job and still on M1 or W1, stop waiting. Check the HMRC app first. If your employment details look correct there but your payslip code has not updated, your employer’s payroll is using stale data, so ask them to apply the code HMRC has issued. If HMRC’s own record looks wrong, call them on 0300 200 3300 and they will correct it and issue the right code to your employer.
You do not usually need form P50 for this situation. P50 is for when you have stopped working entirely and want a refund mid-year. A straightforward job change fixes itself through PAYE once the code catches up.
The mechanics of emergency tax codes are unchanged for 2026/27. The personal allowance is still £12,570, the basic rate is still 20% up to £50,270, and M1 and W1 still mean the same non-cumulative thing they always have.
What has shifted is where you check. HMRC has moved tax code notices online-first, so the quickest way to see your current code, confirm your employments, and track a pending correction is now the HMRC app rather than waiting for a letter. If your August payslip looked light and you want to know why, that is the first place to look.
If you want to see what your take-home should be on a correct cumulative code, at your salary or hourly rate, the NetPay app does the full calculation including National Insurance and pension. Free to download.
See your real take-home pay in seconds
NetPay does this maths for every shift, invoice and payslip, automatically.
Download the app now
M1 and W1 are emergency, non-cumulative tax codes. M1 is for monthly pay, W1 for weekly pay. They tell payroll to tax each pay period on its own, ignoring what you earned or paid earlier in the tax year. They usually appear after a job change when your new employer does not yet have your full pay history, and they often mean you overpay tax for a month or two.
Usually yes. Once HMRC links your new job to your full year-to-date figures, your code switches to a normal cumulative code and payroll refunds any overpayment through your next payslip. If the tax year ends before that happens, HMRC reconciles it and sends the refund afterwards. You can speed it up by giving your employer your P45 or checking your details in the HMRC app.
If your P45 reaches your new employer quickly, the correction often lands within one or two pay cycles, so four to eight weeks is common. If you have no P45 and rely on HMRC updating your record, it can take longer. Logging into the HMRC app and confirming your employment details is the fastest way to prompt the fix.
Often not. Handing your new employer your P45, or completing the new starter checklist accurately, usually resolves it on its own. If you are still on M1 or W1 three months into the job, then it is worth checking the HMRC app and, if your record looks wrong, calling HMRC on 0300 200 3300.
Want to see your actual take-home pay?
NetPay UK works out your real net pay after tax, NI, pension and salary sacrifice, for hourly, shift and variable-income workers. Free to download.
Download the app now
A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (1 August 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
