W1/M1 Emergency Tax Codes Explained: Why You're Taxed Month by Month
Codes ending in W1 or M1 mean you're being taxed on a 'non-cumulative' basis — each payslip treated in isolation rather than tracking your total pay for the year. Here's why this happens and how it usually gets corrected.
Cumulative vs Non-Cumulative Tax Codes
Emergency tax code guide| Cumulative (normal) | Non-cumulative (W1/M1) | |
|---|---|---|
| How pay is assessed | Running total across the whole tax year to date | Each pay period assessed in isolation |
| Unused allowance from quieter months | Carried forward and applied | Not carried forward |
| Typical trigger | Standard, ongoing employment with full information | New job without a P45, unresolved coding query, complex income change |
| Risk of overpayment | Lower | Higher, especially with variable monthly income |
Worked Example: Why Non-Cumulative Taxation Can Overtax You
Suppose your Personal Allowance is £12,570, giving roughly £1,047.50 of tax-free allowance per month on a cumulative basis.
| Month | Pay | Cumulative basis | W1/M1 basis |
|---|---|---|---|
| Month 1 (no income yet — new job starts month 2) | £0 | N/A | N/A |
| Month 2 | £3,000 | Uses month 1 + 2 allowance (£2,095) tax-free, taxes the rest | Only that month's allowance (£1,047.50) applied — more of the £3,000 taxed |
On a cumulative basis, unused allowance from month 1 rolls into month 2's calculation. On a non-cumulative (W1/M1) basis, it doesn't — resulting in more tax deducted in month 2 than would ultimately be owed for the year, an overpayment typically corrected once you move to a cumulative code.
Common Triggers
- Starting a new job without a P45 from your previous employer (or one is provided but received late).
- Complex or changing income situations HMRC hasn't fully processed — multiple jobs, benefits changes, pension income starting.
- A coding change applied only going forward, without HMRC yet reconciling the earlier part of the year.
What to Do About It
- Provide your new employer with a P45 (or complete a starter checklist if you don't have one) as promptly as possible — this is the single most common fix for a new-job emergency code.
- Check your payslip each month to see whether your tax code has switched from W1/M1 to a standard cumulative code.
- If it persists for more than a couple of months, contact HMRC directly to find out what information is outstanding.
- At year-end, check whether a P800 tax calculation shows a refund due — any overpayment caused by an extended period on a non-cumulative code is typically identified and repaid once HMRC reconciles the full year.
Frequently asked questions
Related reading
UK Self Assessment From Scratch — Part 8: After You File
What happens after you submit your Self Assessment return — refunds, balancing payments, amendments, HMRC enquiries, the SA302 for mortgages, and the 5-year record-keeping rule
UK Self Assessment From Scratch — Part 7: Making Tax Digital for Income Tax
Making Tax Digital for Income Tax (MTD ITSA) starts April 2026 for £50k+ self-employed and landlords. Here's what it means, when it applies to you, the software requirements and how it changes Self Assessment forever.
UK Self Assessment From Scratch — Part 6: Payments on Account Explained
How HMRC's payments-on-account system works, why your first January bill is bigger than expected, when to reduce them, and the trap of treating January and July as separate