What M1 and W1 tax codes mean, when they appear, why they cost you money, and how to get your refund in 2026.
If you’ve recently changed jobs and spotted an unfamiliar code on your payslip—something like 1257L M1 or 1257L W1 where you’re used to just 1257L—you’ve likely spotted an emergency tax code. And you’ve also likely just noticed that the tax being deducted looks higher than it should.
You’re not imagining it. Emergency codes cost you real money. Here’s what’s happening, why, and how to get it back.
These codes mean your employer hasn’t yet told HMRC what you earned before you arrived, so they’re calculating your tax each pay period as if this is your first week of the year, rather than adjusting for what you’ve already earned elsewhere.
M1 means monthly non-cumulative (resets every month). W1 means weekly non-cumulative (resets every week). Both are temporary, both cost you money, and both should disappear once the data catches up.
Here’s the mechanism. Normally, a standard tax code like 1257L works cumulatively. Your employer tracks your total earnings from April onwards, protects the personal allowance (£12,570 for 2026/27) spread evenly across the year, and adjusts tax accordingly.
On M1 or W1, payroll ignores everything that came before. Each pay period, the system thinks you’re starting fresh.
Worked example: you left a job on 31 May after earning £13,000, then started a new job on 1 June at £2,500 per month salary. Your new employer doesn’t have your P45 yet, so you’re put on 1257L M1.
June payslip (cumulative would look like this):
But on M1 (what actually happens):
Looks the same so far. But keep going.
July on M1 (non-cumulative reset):
Now compare that to what cumulative would show:
That still looks even. The real damage shows up when you’ve also earned money at your first job before changing over. That’s when the personal allowance gets wasted.
Real overpayment scenario: you earned £10,000 at your first job (used up most of your allowance), then started a new job earning £2,500 per month. Your first employer didn’t send your P45 on time.
Result: you overpay tax every single month you’re on M1 or W1.
You land on an emergency code in three main situations:
Situation 1: Job change without a P45. You leave Job A and start Job B. Your old employer was supposed to give you a P45 (proof of what you’ve earned and taxed so far). If they didn’t, or if it arrived late, Job B’s payroll can’t see your earnings history. So they put you on M1 or W1 as a placeholder while they wait for your data to arrive.
Situation 2: You left before getting a P45. Maybe you were fired, left suddenly, or the P45 got lost in the post. Either way, Job B has no way to know what you earned before.
Situation 3: HMRC’s data is slow. Very occasionally, HMRC’s systems take longer than expected to update. Even if you gave your new employer a P45, the data might not have flowed back to HMRC’s records yet, and payroll might flag the gap as M1 or W1 just to be safe.
Why does it stick around? Because data. HMRC, your old employer, your new employer, and the Startup Checker Service all have to sync up. That usually takes 4-8 weeks. During that time, you’re on M1 or W1.
In April 2026, HMRC switched to a digital-first P2 notice system. Instead of a letter in the post, tax code changes now land in your HMRC online account first. The email notification is easy to miss.
That means if you started a new job in April or May 2026 and ended up on an emergency code, you might not have received a clear notice. The code was updated in HMRC’s system, but it reached your inbox as a background notification rather than an official-looking letter.
Worth checking: if you switched jobs around April 2026 and you’re unsure whether your code has been fixed, log into the HMRC app. It shows the live status.
Three places to check:
1257L M1 or 1257L W1.If your P45 is sitting at home unopened, give it to payroll now. Parts 2 and 3 go to them. Part 1 is for your records. This is the fastest way to get the code corrected.
If you don’t have a P45 (lost, never received, etc.), your new employer can request one from your old employer or use a Starter Checklist if you can’t retrieve the original. Payroll will guide you through this.
Once payroll has the P45 and sends it to HMRC, it usually takes 2-4 weeks for the system to update your code. Your next payslip should show the code changed back to standard (for example, 1257L instead of 1257L M1).
If that doesn’t happen within a month, move to Step 3.
If the code hasn’t corrected itself after a month, or if you need the money before the year ends, claim directly:
Via HMRC app: Log in, go to Income Tax, select the current year, and look for a refund or overpayment option. You may be able to request it straight away.
Via phone: Call 0300 200 3300 (HMRC Income Tax Team). Tell them:
They’ll investigate and either auto-correct your code (you’ll see the refund in your next payslip) or issue a direct refund to your bank account.
Timeline: Direct refunds take 4-6 weeks. During busy periods (January-April), expect 6-8 weeks.
If you want to calculate exactly how much you’ve overpaid, the NetPay app can help you work through the scenario—input your gross pay, the dates you were on M1 or W1, and it’ll show you what you should have paid versus what actually came off. Free to download on iOS and Android.
See your real take-home pay in seconds
NetPay does this maths for every shift, invoice and payslip, automatically.
Download the app now
It depends. If the overpayment is small and happens early in the tax year, HMRC usually corrects it through your payslip within a few months without you doing anything. If the year ends with overpayment still showing (or if you need the money before then), you'll need to claim via the HMRC app or phone them on 0300 200 3300.
Auto-corrections through payroll usually take 2-4 months. If you claim directly with HMRC, expect 4-6 weeks for a refund to appear in your bank account. During peak tax-year periods (January-April), it can take longer.
Yes. The overpayment happens because non-cumulative codes (M1 and W1) treat each pay period as the first of the year. Work out what you should have paid based on your cumulative earnings, then compare to what actually came off. The NetPay calculator can help estimate this if you know your gross pay and number of pay periods so far.
If it's small (under £100) and you started the job recently, it's usually worth waiting a month or two to see if payroll corrects it automatically. If it's substantial or you've been on M1 or W1 for three months or more, contact HMRC directly rather than waiting.
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 (3 June 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
