IsMyPayRight

Guide

Cumulative Tax vs M1/W1 Tax: Why One Payslip Can Look Wrong

If one payslip looks wrong but the annual picture might still settle down later, the real issue is often cumulative versus M1 or W1 treatment.

Reviewed by IsMyPayRight editorial team

Last updated 17 April 2026

Updated for the 2026/27 tax year

Quick answer

Cumulative PAYE uses your year-to-date position. M1 or W1 uses just that one month or week. Cumulative treatment is usually more accurate over the full tax year, while M1 or W1 is usually a temporary emergency basis.

What cumulative tax means

Cumulative PAYE is the normal year-to-date method. Payroll looks at how much pay you have had since 6 April, how much allowance you should have used by now, and how much tax has already been taken.

That makes cumulative tax much better at keeping the full-year picture in line, especially when pay is uneven or a code changes part way through the year.

What M1 and W1 mean

M1 means month 1 and W1 means week 1. GOV.UK treats them as emergency-tax markers, and some software shows X or NONCUM instead.

All of them mean payroll is not looking back across earlier periods when working out the current tax deduction.

Why the same code number can still give a different answer

A code such as 1257L and a code such as 1257L M1 do not behave the same way. The first is usually cumulative. The second uses only one pay period at a time.

That is why people can compare payslips with the same main code number and still get very different tax deductions.

If one payslip looks isolated from the rest of the year

These next pages help you decide whether the issue is emergency treatment, a wrong code, or just a short-lived setup problem.

When M1 or W1 is most visible

The difference is usually clearest after a job change, a late code update, or an irregular pay month. Under cumulative PAYE, the new code can catch up with what should have happened earlier in the year. Under M1 or W1, that catch-up does not happen automatically.

So one payslip can look sharply out of line even though the gross pay is ordinary.

What to do if you want the code back on cumulative

P9X says employers should not carry week 1 or month 1 markings over into the new tax year when copying a code forward. In-year, the usual route is still for HMRC to send the corrected cumulative code once it has the right details.

If the suffix is still there after the information issue has been fixed, ask HMRC to review the code rather than assuming payroll can remove it on its own.

What to check

  • Cumulative tax can refund earlier overpayments automatically.
  • M1 or W1 usually cannot look back and correct earlier months.
  • A code can have the same main number but still behave differently because of the suffix.

What to do next

  • Look for M1, W1, X, or NONCUM on the code.
  • Compare the current code with the latest HMRC notice.
  • Use the checker if you want to see whether the month looks isolated from the year-to-date picture.

Try the tool

Use the checker if you already have a payslip. Use the calculator if you want to model take-home pay or salary-sacrifice changes before payday.

Why you can trust this guide

This guide is maintained by the IsMyPayRight editorial team team and is aligned to the PAYE assumptions used by the calculator and payslip checker.

We write against HMRC rules first, then explain the payroll implications in plain English so the article and the tool stay consistent.

Methodology and sources

UK PAYE guidance content is backed by the same methodology used across the engine and checker.

Common questions

What is the simplest difference between cumulative and M1/W1?
Cumulative looks at the tax year so far. M1 or W1 looks only at the current pay period.
Can M1 or W1 stop an automatic refund on the next payslip?
Yes. Because it does not look back across earlier periods in the same way, it can stop the normal catch-up effect you see under cumulative PAYE.
Does a new tax year remove M1 or W1 automatically?
P9X says employers should not carry week 1 or month 1 markings over when copying a code into the new tax year.

Official sources

We checked the claims in this guide against the official source pages below and the current 2026/27 calculator rules.