Cumulative vs Week 1 / Month 1 tax basis: why your payslip looks different
Last updated: 5 May 2026
TL;DR
Cumulative PAYE looks at year-to-date pay and tax to balance your liability across the whole year. The non-cumulative basis - marked W1, M1 or X on your tax code - taxes each pay period in isolation, ignoring what happened in earlier periods. The codes can be the same; the basis is what changes the maths.
In one sentence
Cumulative PAYE smooths tax across the year; W1/M1/X taxes each period on its own and is normally a temporary state.
What is the cumulative basis?
Cumulative PAYE is the default. Each pay period your employer adds up your year-to-date pay, applies the proportion of personal allowance and band thresholds you have earned so far, calculates the tax that should have been deducted to date, and subtracts what has already been deducted. The result is the tax for this period.
The effect is that pay rises, bonuses, unpaid leave and mid-year allowance changes are smoothed out. If you under-paid earlier in the year, the cumulative system catches up; if you over-paid, you receive a partial refund through reduced PAYE on the next payslip. Most people on a stable single job stay on the cumulative basis all year - for example, code 1257L with no suffix.
What is week 1 / month 1 (W1 / M1 / X)?
Non-cumulative PAYE - signalled by a W1, M1 or X suffix - calculates each pay period in isolation. Year-to-date pay is ignored. The system treats every payday as if it were the first of the year and applies one period's worth of allowance and band thresholds. The same payslip would produce the same tax whether it was your first month in the role or your eleventh.
Non-cumulative codes typically appear when HMRC does not yet have full year-to-date information for you. Common triggers: starting a new job without a P45, a tax-code change partway through the year, returning from a career break, or a multi-job situation that HMRC needs to reconcile. The code usually reverts to cumulative once HMRC has the complete picture, often after the next coding notice. Our emergency-tax explainer goes deeper.
Side-by-side comparison
| Dimension | Cumulative | Week 1 / Month 1 (W1/M1/X) |
|---|---|---|
| Code marker | None (e.g. 1257L) | W1, M1 or X suffix (e.g. 1257L W1) |
| What it considers | Year-to-date pay and tax | This period only |
| Catches up over- / under-payments? | Yes, automatically | No - frozen each period |
| Typical scenario | Stable single job, full P45 on file | New starter, mid-year change, missing P45 |
| Personal allowance applied each period | Pro-rata up to year-to-date | One period's worth (e.g. £1,047.50/month for 1257L) |
| Higher-rate threshold each period | Pro-rata YTD (£50,270 ÷ 12 cumulatively) | One period's worth (£4,189.17 / month) |
| Effect of a one-off bonus | Smoothed across remaining year | Taxed in the period received with no smoothing |
| Year-end reconciliation | Usually balanced | May leave an over- / under-payment for HMRC to settle |
| How to switch | n/a | HMRC reissues a cumulative code (often after P45 / PTA update) |
| Permanent or temporary? | Permanent default | Temporary - should resolve within a coding cycle |
Suspect you are stuck on W1/M1/X? Run a free payslip check - we will flag the suffix and explain the most likely cause.
Worked example: same code, different basis
Sam earns £36,000 a year on code 1257L. Pay is monthly. We'll use 2026/27 thresholds: £12,570 personal allowance, 20% basic rate up to £50,270.
Cumulative basis. By month 6, Sam's YTD gross is £18,000. Six months of allowance is £6,285, so YTD taxable pay is £11,715. Tax due YTD = £11,715 × 20% = £2,343. The previous five months already deducted, say, £1,952.50 cumulatively, so this month's tax = £2,343 − £1,952.50 = £390.50. Each month is roughly the same; the system smooths.
W1 / M1 basis. Each month is calculated standalone. One month of allowance is £1,047.50, gross of £3,000 minus £1,047.50 = £1,952.50 taxable. Tax = £1,952.50 × 20% = £390.50. In Sam's steady example the monthly tax happens to be the same. Now suppose Sam took unpaid leave in month 3, earning zero. On the cumulative basis, month 4 would receive a refund as the YTD allowance catches up. On W1 / M1, month 3 still gets nothing back, and month 4 simply pays its own period's tax. That mismatch is what makes W1 / M1 over- and under-collect across the year.
When you might see W1 / M1 / X
- You started a new job and had no P45 from a previous employer.
- Your tax code changed mid-year because of a benefit-in-kind or marriage-allowance update.
- You took a career break and re-joined PAYE.
- HMRC issued a temporary code while reconciling multiple income sources.
- You moved into or out of Scotland and the prefix is being updated.
Pros and cons
Cumulative
Pros: self-balancing, smooths anomalies, the right default for most employees. Year-end almost always reconciles cleanly.
Cons: a single sharp pay change (large bonus, retroactive pay rise) is smoothed across the rest of the year, which can feel unintuitive on the payslip.
Week 1 / Month 1
Pros: simple per-period maths; useful as a temporary measure when HMRC does not have full year-to-date data; protects against retrospective surprises in some edge cases.
Cons: can over- or under-collect tax; doesn't self-balance; may leave a year-end discrepancy that requires a P800 reconciliation.
What to do if you think the basis is wrong
- Confirm the suffix on your latest payslip code (W1, M1 or X).
- Check your HMRC Personal Tax Account - does the latest P2 coding notice show a non-cumulative basis too?
- If you joined recently, send your P45 to payroll. HMRC normally reissues a cumulative code on the next pay run.
- If the code looks wrong altogether, see our wrong-tax-code checklist.
Frequently asked questions
How do I tell which basis my payslip is on?
Look at the tax code. A code with a W1, M1 or X suffix (for example 1257L W1) is non-cumulative. A code without that suffix is cumulative.
Is week 1 / month 1 the same as emergency tax?
Often, yes. Emergency tax usually means 1257L on a non-cumulative basis (W1/M1/X). Some emergency situations use 0T on a non-cumulative basis instead, which gives no personal allowance until reconciled.
Will HMRC automatically refund overpayments at year-end?
If you stay on a non-cumulative code, you may not get the year-to-date balancing. HMRC typically reconciles after the tax year via a P800 if amounts are owed, but moving back to a cumulative code is the cleaner fix.
Can I ask payroll to switch me back to cumulative?
Payroll cannot do this on its own - it has to be HMRC's tax-code instruction. Updating your details with HMRC (often through the Personal Tax Account or by sending a P45) usually triggers a new cumulative code on the next pay run.
Related pages
- Run a free payslip check
- Guide: PAYE cumulative vs non-cumulative
- Guide: emergency tax explained
- Guide: tax codes explained
- Tax-code library: 1257L
- Tax-code library: 0T
- Check a P2 coding notice
Educational content based on HMRC PAYE guidance for 2026/27. Not regulated tax advice.
See if your latest payslip is on the right basis - in plain English.