Emergency tax codes: W1, M1 and X

This site is an independent UK tax-code explainer. We explain the payroll rules but cannot access your records or promise a specific outcome.

What the markers mean

When you start a new job or your circumstances change unexpectedly, you may see W, M or X markers on your payslip alongside your code. These markers are instructions to the payroll software. They tell the employer to change how your tax is calculated for that specific pay period.

Written out, the markers this site recognises are W1, M1, X and NONCUM, which is the list HMRC publishes. W1 names a week and M1 a month; HMRC sets all four down together as markers for the one statutory basis, and this site does not rank them beyond that. What matters for your pay is the same whichever of them you have: none of them changes which code you are on. A marker on a code is the code plus a basis, not a different code.

A payslip does not always leave a space between the code and its marker, and it does not need to. 1257L W1 and 1257LW1 are the same string as far as this site is concerned: the checker on this page’s site strips spaces before it looks a code up, so either spelling decodes to the same answer.

These markers are administrative. HMRC’s payslip markers appear nowhere in the legislation. They are a shorthand for a specific legal process. The statutory concept driving these markers is the non-cumulative basis under regs 26–31.

The emergency codes for the 2026/27 tax year

HMRC publishes these as the emergency codes for the year: 1257L W1, 1257L M1, 1257L X. Each one is the standard code with a marker after it, which is why the number in an emergency code is the same number most people on one job carry — the emergency part is the basis, not the allowance. The tax-year page lists them beside the standard code and the three rate scales.

A marker can sit on any code, not only the standard one. A flat-rate or no-allowance code can carry one too — 0T and BR are the two this site sees searched with a marker most often — and each of those pages states the regulation that governs that particular code.

Cumulative and non-cumulative, and why the difference costs money

By default, payroll operates on the cumulative basis under reg 22. On a cumulative basis, the employer looks at your total pay and total tax for the entire tax year to date. The calculation smooths out fluctuations in your earnings and ensures that any unused allowance from previous months is applied to your current pay.

When the non-cumulative basis applies, the cumulative history is ignored. Under reg 27(1), on the non-cumulative basis the employer deducts the tax that would have been deductible “if the payment had been made on the first day of the tax year”.

In practice, the employer treats the current pay period as an isolated event. They apply exactly a single period’s worth of your free pay to that specific payslip. They do not look at what you earned last month, nor do they look at what tax you have already paid. Every single payday is calculated as if it were the initial payday of the year.

Why it overtaxes a mid-year start

This explains why the non-cumulative basis overtaxes someone who started mid-year. If you begin employment halfway through the year and have not worked previously, you possess unused allowance from the preceding months. Under the normal cumulative basis, that accrued allowance would offset your new earnings, resulting in a low deduction on your initial payday. However, because reg 27(1) treats the payment as if it were the first day of the tax year, that historical allowance is ignored. You receive only a single period’s allowance, causing more tax to be deducted than necessary.

Why it undertaxes a second job

Conversely, this mechanism undertaxes an additional job. If you take on another job and are placed on a non-cumulative basis, the new employer calculates the tax as an isolated event. They apply the free pay for that period without realising you have already used your allowance at your main job. This duplicates the allowance, resulting in a deduction that is too small.

How an emergency code ends

The non-cumulative basis is a temporary measure. It ends when HMRC gathers your complete information and determines a new code. HMRC issues a revised instruction to your employer on a cumulative basis.

On the next payday following the new instruction, the employer reverts to reg 22. The software looks at the whole year again, applies your full allowance to date, and subtracts the tax you have paid so far. The deduction adjusts automatically.

It is important to remember that the payroll software operates automatically based on these instructions. The software does not know your personal circumstances; it only knows the data HMRC has transmitted. If the data transmitted is outdated, the deduction will be mathematically accurate according to the instruction, but factually wrong for your life. This is why maintaining accurate records with the authorities is the only way to ensure the system operates as intended.

The only place a person’s actual code and its basis can be viewed is HMRC’s authenticated Personal Tax Account. This site cannot know it, and does not try to. You must log in to your account to establish if HMRC has the information required to move you off the non-cumulative basis.

Common questions