M1 is an emergency tax code. Tax is calculated on your monthly pay only -- not your year-to-date earnings -- which usually means you overpay and should request a correction.
M1 is an emergency tax code that instructs your employer to calculate your income tax on a non-cumulative, month-by-month basis. Every month is treated as month 1 of the tax year. No unused Personal Allowance carries forward from previous months, which means most people on M1 overpay tax -- often by several hundred pounds over the course of a few months.
Under a normal cumulative tax code like 1257L, your £12,570 Personal Allowance is spread evenly across 12 months. Each month you receive £1,047.50 of tax-free allowance. Crucially, if you did not work for the first three months of the tax year, those three months of unused allowance (£3,142.50) carry forward and reduce your tax when you start earning. This is the cumulative system. It ensures you pay the correct total tax by year end.
M1 breaks that mechanism. Your employer applies exactly £1,047.50 of allowance to each month, ignoring everything that came before. If you had three months of zero earnings, those three months of allowance vanish. You are taxed as if the year began on the day you started.
Worked example: £3,500 monthly salary, started in month 4 (July)
Under a normal cumulative code (1257L):
Under M1 (non-cumulative):
Difference after two months: cumulative tax = £352.50. M1 tax = £981.00. Overpayment = £628.50.
That overpayment grows with every month M1 remains on your code. By year end, someone who started mid-year on M1 can easily overpay by £1,000 or more.
M1 is assigned automatically when your employer's payroll system lacks the information needed to run a cumulative code. You did not choose it and you cannot select it. The most common reasons are:
What it looks like on your payslip: You will typically see your tax code followed by M1 -- for example, 1257L M1. Some payroll systems display NONCUM (non-cumulative) or NIC instead. All mean the same thing: your tax is being calculated month by month, not cumulatively. If you are paid weekly rather than monthly, you would see W1 instead of M1.
If you started your job in any month other than April (month 1 of the tax year), you are almost certainly overpaying. The overpayment happens because M1 does not give you credit for the months when you had no earnings or lower earnings.
Quick check:
If the deducted amount is higher, M1 is costing you money.
Higher earners feel it more. If your monthly salary puts you close to the 40% threshold (£50,270 annually), M1 can push individual months into the higher rate band even though your annual income would stay within basic rate. A cumulative code would spread the bands across the year and keep you at 20%.
M1 is supposed to be temporary. Follow these steps to get a correct cumulative code:
Step 1: Complete the Starter Checklist. Your employer should give you this form when you start. It replaced the old P46 and has three statements:
Read each statement carefully. The wrong choice can create different problems: Statement A when you have a second job splits your allowance across two employers.
Step 2: Hand over your P45. If you have one from your previous employer, give it to your new employer's payroll team on your first day. This is the single fastest way to get off M1 because it gives payroll your exact year-to-date figures.
Step 3: Check your HMRC Personal Tax Account. Log in at gov.uk/personal-tax-account and verify that HMRC has your correct employment details. Update anything that is wrong or missing.
Step 4: Call HMRC if it persists. If M1 is still showing after your second payslip, phone the Income Tax helpline on 0300 200 3300 (Monday to Friday, 8am to 6pm). They can issue a corrected code to your employer within days.
How long does the fix take? Once HMRC has the right information, a new code is typically sent to your employer within 2-3 weeks. Your employer switches to a cumulative basis from your next pay run.
Yes. Overpaid tax on M1 is always refundable. The method depends on when the correction happens:
During the tax year (most common). When your employer receives a cumulative code from HMRC, they recalculate your year-to-date tax from 6 April. The difference between what you paid on M1 and what you should have paid cumulatively is refunded in your next wage packet. Expect an unusually large net pay that month.
After the tax year ends. If M1 runs for the whole year, HMRC performs an automatic reconciliation after 5 April. They send a P800 tax calculation (usually between June and November). If you overpaid, you can claim the refund online through your Personal Tax Account or wait for a cheque, which arrives within 60 days.
You do not have to wait. Contact HMRC at any time to request a review. If they confirm an overpayment, they can issue a refund directly to your bank account or adjust your tax code so you pay less for the remaining months.
M1 and W1 are identical in how they work. The only difference is the pay period:
| Feature | M1 (Month 1) | W1 (Week 1) |
|---|---|---|
| Pay frequency | Monthly | Weekly |
| Allowance per period | £1,047.50 | £241.73 |
| Non-cumulative? | Yes | Yes |
| Carry forward? | No | No |
| Fix process | Starter Checklist + P45 | Starter Checklist + P45 |
| Refund method | Same | Same |
Most UK employees are paid monthly, which is why M1 gets more search volume (12,100/month) than W1 (9,900/month). If your payslip says NONCUM without specifying M1 or W1, check your pay frequency. Monthly pay = M1. Weekly pay = W1.
Both codes appear alongside your tax code number. 1257L M1 and 1257L W1 mean the same thing: you have the correct Personal Allowance, but it is being applied non-cumulatively.
| Code | Allowance? | How it works | Cumulative? |
|---|---|---|---|
| M1 | Yes (monthly fraction: £1,047.50) | Tax on this month's pay only, no carry-forward | No |
| 0T | No allowance at all | All income taxed through the bands (20%, 40%, 45%) | Usually yes |
| BR | No allowance on this source | Flat 20% on all income from this employer | Yes |
M1 is generally better than 0T because you at least get a monthly slice of your Personal Allowance. 0T gives you nothing. BR is different again: it applies a flat 20% regardless of how much you earn, which is correct for a second job where your main job already uses your allowance.
1. Assuming M1 will fix itself. It will not. HMRC needs a Starter Checklist or P45 to issue a cumulative code. If you do nothing, M1 stays on your record indefinitely.
2. Not providing your P45 to your new employer. The P45 is the key document that unlocks cumulative taxation. Even if you think HMRC will sort it out, hand over the P45 on day one. Payroll cannot back-date the correction to your start date if they receive it months later.
3. Confusing M1 with the M tax code (Marriage Allowance). M1 is an emergency non-cumulative suffix. The M tax code letter means you receive the Marriage Allowance transfer from your partner. They are completely unrelated. If your code says 1257M, that is Marriage Allowance. If it says 1257L M1, that is emergency tax.
4. Ignoring the overpayment because the amount seems small. Even £50-£100 per month adds up. Over 6 months, a £75/month overpayment costs £450. That money sits with HMRC earning you nothing when it could be in your account.
5. Choosing the wrong Starter Checklist statement. Selecting Statement A ("first job since April") when you had earlier employment in the tax year can lead to your Personal Allowance being applied twice, creating an underpayment that HMRC will claw back later.
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