A Week 1 or Month 1 tax code is a non-cumulative code that tells your employer to work out your income tax on each pay period on its own, ignoring what you’ve earned or paid in tax earlier in the year. Your £12,570 personal allowance for 2026-27 gets sliced into equal weekly or monthly portions, and only that slice is applied to the pay in front of the payroll system.1UK Parliament. Direct Taxes: Rates and Allowances for 2026/27 HMRC uses it as a temporary measure when it doesn’t have enough information about your earnings history to tax you cumulatively. Most people on it overpay, and there are straightforward ways to switch back and reclaim the difference.
How to Spot It on Your Payslip
Look at the tax code on your payslip or P45. If a suffix follows the code, that’s the flag:
- W1 if you’re paid weekly (for example, 1257L W1)
- M1 if you’re paid monthly (for example, 1257L M1)
- X if your pay dates vary
- NONCUM in some payroll software instead of a letter suffix
If none of those appear, you’re on a standard cumulative code and your employer is already factoring in your year-to-date earnings.2GOV.UK. Tax Codes: Emergency Tax Codes The number in the code (like 1257) represents your annual tax-free allowance with the last digit removed, so 1257 means £12,570. The letter after it, usually L, indicates your allowance category. It’s the W1, M1, or X on the end that changes how the calculation actually runs.
What Non-Cumulative Really Means for Your Pay
A standard cumulative code tracks every pound you’ve earned since 6 April and every pound of tax you’ve paid. Each payday it recalculates your total liability for the year so far. If you earned less in earlier months, the system catches up and refunds the excess through a larger net payment.
A non-cumulative code can’t do this. Every payslip is treated as if it exists on its own, with no memory of what came before.2GOV.UK. Tax Codes: Emergency Tax Codes Your personal allowance is split into equal portions, one-twelfth per month or one-fifty-second per week, and only that slice is applied to your gross pay each period. Whatever remains gets taxed.
This matters most when your income fluctuates. Say you earn £4,000 one month and £1,500 the next. Under a cumulative code, the lower month partially offsets the higher one and the totals balance. Under a W1 or M1 code, the £4,000 month is taxed as if you earn £4,000 every month, and the £1,500 month is taxed as if you earn £1,500 every month. Neither calculation knows about the other.
In most cases, this means you overpay. The system can’t account for weeks or months earlier in the tax year when you earned less or nothing at all, so you lose the benefit of unused personal allowance from those periods. Someone who started a new job in September has already “missed” five months of allowance that a cumulative code would fold back in. On a W1 or M1 code, those months are invisible.
The reverse can also happen. If you earned heavily earlier in the year and then moved to a lower-paying role, a non-cumulative code could temporarily undertax you, because the new employer doesn’t see your earlier high earnings. That gap catches up eventually, either through a corrected code or an end-of-year reconciliation.
Why You Ended Up on It
The most common trigger is starting a new job without handing your P45 to your employer. Your P45 carries your year-to-date pay and tax figures from your previous role, and without it your new employer has no baseline to work from.2GOV.UK. Tax Codes: Emergency Tax Codes This happens regularly to people entering work for the first time, returning after a career break, or moving from self-employment back to a salaried job.
You can also land on this code by selecting Statement B on the Starter Checklist your new employer asks you to complete. Statement B applies when you’ve had another job since 6 April but don’t have a P45, or when you’ve received Jobseeker’s Allowance, Employment and Support Allowance, or Incapacity Benefit during the tax year. HMRC’s instructions are explicit that Statement B means applying the personal allowance on a Week 1/Month 1 basis.3HM Revenue and Customs. Starter Checklist If a different statement genuinely applied to you and you picked B out of uncertainty, you’ve been placed on a non-cumulative code unnecessarily.
Other situations that can trigger it include starting to receive company benefits like a company car or private health insurance, or beginning to draw the State Pension alongside employment income. In both cases HMRC may temporarily switch you to a non-cumulative basis while it recalculates your overall position.2GOV.UK. Tax Codes: Emergency Tax Codes
How to Switch to a Cumulative Code
The fastest route is giving your new employer your P45 from your previous job. The P45 shows your total pay and tax for the year so far, which lets payroll slot you into the cumulative system straight away.4GOV.UK. Your P45, P60 and P11D Form If your old employer hasn’t sent one, ask them directly. They’re required to issue it.
No P45 available? Contact HMRC yourself. The simplest way is through your Personal Tax Account online, where you can check your current tax code, see the employment information HMRC holds, and report changes.5GOV.UK. Check Your Income Tax for the Current Year You’ll need to sign in with a Government Gateway account and may need photo ID to verify who you are. You can also call HMRC’s income tax helpline on 0300 200 3300, or +44 135 535 9022 from outside the UK.6GOV.UK. Income Tax: Enquiries
Once HMRC updates your records, it sends your employer a new tax code through a coding notice, sometimes called a P6.7GOV.UK. Understanding Your Employees’ Tax Codes Your employer must apply the new code before your next payday, or by your third payday if you’re paid weekly. When the cumulative code takes effect, payroll recalculates your tax for the whole year to date and adjusts your net pay.8GOV.UK. Tax Codes: If You’ve Paid Too Much or Too Little Tax
Getting Overpaid Tax Back
If your code is corrected during the tax year, the overpayment usually sorts itself out automatically. Your employer’s payroll system reworks your year-to-date position under the new cumulative code and refunds the excess through your pay. You’ll see a noticeably larger net payment on the first payslip after the switch.8GOV.UK. Tax Codes: If You’ve Paid Too Much or Too Little Tax
If you stay on a non-cumulative code through the end of the tax year on 5 April, HMRC’s P800 reconciliation should catch the discrepancy after the year closes. HMRC compares what you actually earned against what you paid in tax and issues any refund owed. This can take several months after the year ends, so acting during the year is always faster.
If you believe you’ve overpaid and haven’t seen either a payroll adjustment or a P800 letter, use the tax refund checker at gov.uk/claim-tax-refund to see whether you’re eligible and find the right process for your situation.9GOV.UK. Check How to Claim a Tax Refund The sooner you provide the right documents or contact HMRC, the sooner the money comes back to you.