Tax codes one by one
W1, M1 and X tax codes
The endings that switch off the running total: what they do to your tax, and when they go away.
Tax on this payslip (month 8)
£230.20
Worked out on this pay alone.
| Pay to date | £2,200.00 |
| Free pay to date | £1,048.26 |
| Taxable pay to date | £1,151.74 |
| Tax due to date | £230.20 |
| With 1257L | £230.20 |
HMRC’s cumulative method for 2026 to 2027. A real payslip can differ if your pay changed during the year or a past employer’s figures were used.
W1, M1 and X are endings, not codes of their own. Added after a code such as 1257L, they tell the payroll to work out tax on a week 1 or month 1 basis: each pay is treated on its own, as if it were the first pay of the tax year, with one week's or one month's share of the allowance and the bands. GOV.UK calls these emergency tax codes. W1 is used for weekly pay, M1 for monthly pay and X when pay dates vary; some payroll software prints NONCUM instead. The normal method is cumulative: pay and allowance are added up from 6 April, so a month with low pay or a late start gives the unused allowance back later. On the emergency basis that never happens. Someone starting in November on £2,200 a month pays £230.20 each month on 1257L M1; after two months on it, the switch to a cumulative 1257L returns £460.40 through the next payslip. For 2026 to 2027, the emergency code is 1257L with W1, M1 or X. It usually lasts until HMRC has your details, but can stay until the end of the tax year.
Month 1 basis against cumulative
First pay, 1257L M1
£230.20
| First pay, 1257L cumulative | £0.00 |
| Difference on that pay | £230.20 |
| Free pay used by cumulative | £8,386.08 |
Cumulative counts the allowance of the months before the job started.
Rules read on HMRC’s texts by Radif Partners · Editorial policy · Method
Cumulative against week 1 or month 1
HMRC's PAYE specification describes two routines. In the cumulative one, the payroll adds this pay to all pay since 6 April, subtracts the free pay for the same number of weeks or months, works out the tax due to date and takes off the tax already paid. In the week 1 or month 1 routine, set out in section 8 of the specification, each payment is handled in isolation: the free pay for week 1 or month 1 is subtracted from this pay alone, and the tax is worked out as for the first pay of the year.
The employer guidance puts it in one line: calculate the tax only on what the employee is paid in the current period, not the whole year. With code 1257L, that means £1,048.26 of free pay in any month and £241.92 in any week, no matter how many months have gone by.
What month 1 costs a late starter
The table follows someone who had no income earlier in the year, starts a job on £2,200 a month, spends two months on 1257L M1 and then receives a cumulative 1257L. The last column is the tax handed back on the first payslip with the new code.
| First pay in | Tax taken on 1257L M1 | Due on cumulative 1257L | Comes back |
|---|---|---|---|
| July | £460.40 | £0.00 | £460.40 |
| September | £460.40 | £0.00 | £460.40 |
| November | £460.40 | £0.00 | £460.40 |
| January | £460.40 | £0.00 | £460.40 |
The later the start, the larger the refund, because cumulative tax counts the allowance of every month since April. A person who starts in April loses nothing, since there are no earlier months to count.
What comes back after an emergency code
Should come back
£460.40
| Taken on 1257L M1 | £460.40 |
| Due with 1257L | £0.00 |
| Comes back in | January pay |
Assumes no other job earlier in the tax year.
When an employer uses the emergency basis
- A new job without a P45. On the starter checklist, statement B, for someone who has had another job or certain benefits since 6 April, gives the current allowance on a week 1 or month 1 basis.
- A new employer without your details. GOV.UK says the code is usually temporary and HMRC updates it once it has the information from both employers.
- Company benefits or the State Pension starting. GOV.UK describes an emergency code in that case as normal; it can last until 5 April.
- Tax still owed. If not enough tax has been paid, you can stay on the emergency code until the right amount for the year has been collected.
Rules that only apply on the emergency basis
Three details from the specification are easy to miss. First, with a K code on a week 1 or month 1 basis, any tax held back by the limit of 50% of pay is simply lost to the calculation: it is not carried to the next period and does not appear on the P45 or P60. Second, when HMRC raises codes in the middle of the year under a general notice, employees whose code runs on a week 1 or month 1 basis get no refund from that change. Third, at the turn of the year the ending is dropped: HMRC's P9X notice tells employers not to carry any week 1 or month 1 marking into the new tax year, and every suffix code restarts on a cumulative basis on 6 April.
If the ending is still on your payslip more than 35 days after you started, check your employments and estimated pay in your HMRC online account. The emergency codes for 2026 to 2027 are listed in the rates and thresholds for employers.