Tax Code 1257L W1/M1: Overpayments and How to Fix It

If your payslip shows tax code 1257L W1/M1 (sometimes written as 1257L with a zero, or marked NONCUM), your employer is deducting Income Tax on a non-cumulative basis. The 1257L part reflects the standard £12,570 Personal Allowance. The W1 (weekly) or M1 (monthly) marker tells payroll to treat each pay period as if it were the first of the tax year, ignoring what you earned and paid in tax before. It’s an emergency code, it usually means you’re paying more tax than you owe, and it’s straightforward to fix.¹

What the 1257L Part Means

The number is your tax-free Personal Allowance divided by ten. Multiply 1257 by ten and you get £12,570, the amount you can earn each year before Income Tax applies.² The “L” confirms you get the standard allowance with no special adjustments.³

The Personal Allowance has been frozen at £12,570 since April 2021, and legislation now maintains that freeze through April 2031.⁴ So 1257L itself is the code most employees should be on. The problem sits in the marker that follows it.

What the W1/M1 Marker Changes

Under the normal cumulative method, payroll software adds up everything you’ve earned since 6 April and compares it against the tax you’ve already paid that year. If you were overtaxed in one month, the system corrects it the next. The W1, M1, or zero marker switches that off. Each pay period is treated in isolation, as if it’s the very first week or month of the tax year, with no reference to what came before.¹

Your £12,570 allowance gets sliced equally across every pay period. Paid monthly, you receive roughly £1,048 tax-free each month. Paid weekly, about £242.⁵ Anything above that slice is taxed at the applicable rate: 20% within the basic rate band, 40% above £50,270, and 45% above £125,140.²

Because each period stands alone, the payroll system can’t refund tax overpaid in earlier months. The overpayment doesn’t disappear, but you won’t see it corrected on your payslip until either your code changes or the tax year ends.

Why HMRC Put You on This Code

HMRC applies an emergency code when it doesn’t have enough information to calculate your tax accurately for the year. The most common trigger is starting a new job without giving your P45 to your new employer.¹ Without that document, payroll has no record of your earlier earnings or tax paid, so it defaults to the non-cumulative basis.

Other common triggers:

Your first ever job, where there’s no employment history and the starter checklist has to fill in until HMRC catches up.⁶
Taking on a second job before HMRC has allocated your allowance between employers.
Returning to the UK after working abroad, where HMRC may not have current records.
Starting to draw a pension from a new provider that doesn’t yet have your tax details.

The code is meant to be temporary. It stays in place until HMRC sends your employer an updated coding notice, sometimes called a P6.⁷

Are You Actually Overpaying?

Usually, yes. The emergency code isn’t designed to overtax you on purpose, but the lack of year-to-date balancing creates a structural bias.

Take someone who was unemployed from April to August and started a new job in September. Under a cumulative code, payroll would recognise five months of unused Personal Allowance and tax the September pay very lightly. Under 1257L W1/M1, September is treated as if you’ve been earning at the same rate all year, and you get just one month’s worth of tax-free pay.

The reverse case is rare but possible. If you earned heavily early in the year and moved to a lower-paid role, the non-cumulative code can work in your favour by not trying to claw back an earlier underpayment. In practice, most people on emergency codes end up overpaying.

How to Get Your Code Corrected

The fastest route is to hand your new employer your P45 from your previous job. It carries your year-to-date earnings and tax paid, which lets payroll put you on a cumulative code immediately. If your previous employer hasn’t given you a P45, chase them for it.¹

If you don’t have a P45, or the marker is still there weeks later, update your details directly through HMRC’s “Check your Income Tax” online service. Once signed in, you can review your tax code, update your income details from jobs and pensions, and tell HMRC about anything that affects the code.⁸ You can also call the Income Tax helpline on 0300 200 3300, open Monday to Friday, 8am to 6pm.⁹

Have your National Insurance number ready, along with your current employer’s PAYE reference (on your payslip). If you have a P45 or P60, keep them handy too, because HMRC will want your year-to-date earnings. Once HMRC processes the update, it sends a new coding notice to your employer electronically, and the corrected code appears on a future payslip. Any tax overpaid in earlier months gets refunded through your pay once the cumulative code takes effect.

What Happens if You Leave It Until Year-End

If the emergency code stays in place all year and you’ve overpaid, HMRC has a safety net. After each tax year ends on 5 April, HMRC reconciles the tax you paid against what you actually owed. Where the numbers don’t match, it sends a P800 tax calculation letter, typically between June and November.¹⁰

If you’re owed a refund, the P800 explains how to claim it. Online claims through your Personal Tax Account are usually paid within five working days. If HMRC sends a cheque, allow around 14 days from the date on the letter.¹⁰ Waiting for reconciliation works, but it ties up your money for months. Getting the code corrected during the year puts the difference back in your pay packet straight away.

If You Live in Scotland or Wales

Scottish taxpayers see an “S” prefix, so the emergency code appears as S1257L W1 or S1257L M1.¹¹ The £12,570 allowance is the same, but Scotland sets its own rates and bands. For 2025-26 there are six Scottish rates, from a 19% starter rate up to a 48% top rate on income above £125,140.¹²

Welsh taxpayers get a “C” prefix, so the standard code reads C1257L.³ Welsh rates currently mirror those in England, so the prefix doesn’t change what you pay. If your code has the wrong prefix or is missing one, contact HMRC to update your address details.

1
GOV.UK. Emergency Tax Codes
2
GOV.UK. Income Tax Rates and Personal Allowances
3
GOV.UK. What Your Tax Code Means
4
GOV.UK. Income Tax: Maintaining the Personal Allowance and the Basic Rate Limit
5
Low Incomes Tax Reform Group. Tax Codes: What Employers Need to Know
6
GOV.UK. Starter Checklist if You’re Starting a New Job
7
GOV.UK. Understanding Your Employees’ Tax Codes
8
GOV.UK. Check Your Income Tax for the Current Year
9
GOV.UK. Income Tax: Enquiries
10
GOV.UK. If Your Tax Calculation Letter (P800) Says You’re Due a Refund
11
GOV.UK. Coding: General Principles: Scottish Income Tax / Welsh
12
mygov.scot. Scottish Income Tax