Emergency tax codes explained: W1, M1 and X

The standard tax code for the 2026-27 tax year is 1257L, built on a Personal Allowance of £12,570 (gov.uk). An emergency tax code strips the cumulative logic out of that figure, so a new starter can lose part of a refund they are owed, or pay more tax than they should, until the code is corrected (gov.uk).

Emergency codes are common, temporary and almost always resolved within a few weeks under Real Time Information. They surface most often when an employee changes jobs without handing over a P45, when company benefits start, or when the State Pension comes into payment (gov.uk). For the employer running payroll, an emergency code is not an error to fix by guessing a different one. It is a defined starting position that HMRC replaces once it has the right information.

This guide sets out what an emergency tax code is, how to read the W1, M1 and X markers on a payslip, why an employee ends up on one, how payroll software applies it on a non-cumulative basis, and the exact steps that move an employee back onto the correct cumulative code and release any refund.

Key takeaways

• Emergency tax codes end in W1, M1 or X, and some payslips show NONCUM instead, depending on the payroll software (gov.uk).
• An emergency code uses the standard Personal Allowance of £12,570 but applies it to each pay period in isolation, with no running total for the year (gov.uk).
• The most frequent trigger is a new starter whose employer does not yet have their previous pay and tax details (gov.uk).
• HMRC usually issues the correct code once it receives details through RTI, which can take up to 35 days from the start date (gov.uk).
• Any tax overpaid under an emergency code is refunded, either automatically through payroll once a cumulative code arrives, or afterwards by HMRC (gov.uk).

What an emergency tax code is

A tax code tells an employer or pension provider how much tax-free income a person is entitled to in a tax year, and therefore how much tax to deduct from each payment (gov.uk). An emergency tax code is a temporary version of that instruction, used when the payroll does not yet hold enough information to apply the employee's full, cumulative allowance correctly.

Emergency codes are marked by a suffix after the main code. The markers are W1 (week 1), M1 (month 1) and X, and some payroll systems display NONCUM on the payslip to signal the same thing (gov.uk). The core number stays the same as the standard code, so a weekly-paid employee might see 1257L W1, a monthly-paid employee on Scottish rates might see S875L M1, and someone with variable pay dates might see C663L X (gov.uk).

The difference an emergency code makes is not the size of the allowance but the way it is applied. A normal code spreads the £12,570 Personal Allowance evenly across the year and keeps a running total, so earlier overpayments can be refunded automatically. An emergency code gives the same slice of allowance each pay period but ignores everything that came before it, which is why it can produce the wrong result for an employee who started partway through the year (gov.uk).

How to read a tax code

Before an employer can tell whether a code is an emergency one, it helps to read the code in its two parts: a number and one or more letters. The number sets the allowance; the letter explains the employee's situation (gov.uk).

What the number means

The number in a tax code reflects the tax-free income the employee is entitled to from that job or pension in the tax year (gov.uk). HMRC starts with the Personal Allowance, subtracts any untaxed income or deductions, and then drops the final digit, which is why a £12,570 allowance produces the code number 1257 (gov.uk).

For the 2026-27 tax year the standard Personal Allowance is £12,570, giving the familiar 1257L for most employees on a single job (gov.uk). Income above that allowance is taxed at 20% up to £50,270, at 40% up to £125,140, and at 45% above that in England, Wales and Northern Ireland (gov.uk). Scotland runs its own band structure on top of the same allowance (gov.uk). Because this tax is collected through the employer's payroll, an employee's take-home pay is governed by the code applied each payday rather than by a year-end return, a distinction explored in the comparison of PAYE and self-assessment.

What the letters mean

The letter is where an employer learns whether a code is standard, flat-rate or an emergency one. The table below sets out the letters HMRC uses and what each signals (gov.uk) (gov.uk).

The S and C prefixes matter because an emergency code keeps the employee's jurisdiction: a Scottish taxpayer on an emergency code still sees the S prefix, and a Welsh taxpayer still sees C (gov.uk). The 0T code deserves particular attention, because it removes the allowance entirely and is frequently applied to a new starter whose details are missing (gov.uk).

Why an employee ends up on an emergency code

An emergency code is a response to missing or incomplete information, not a penalty. Understanding the trigger tells the employer how quickly it will clear (gov.uk).

Starting a new job without a P45

The most common trigger is a new starter whose employer does not have their previous income and tax details for the year (gov.uk). The P45 from a previous employer carries the year-to-date pay, tax and the code in use, and handing it over lets the new employer apply the correct cumulative figures from the first payday (gov.uk). Without it, the employer collects the information through the starter checklist and reports the new employee on the first Full Payment Submission, but the code applied in the meantime is usually an emergency one (gov.uk).

Giving the new employer a P45 can remove the emergency code, because it supplies the figures the payroll needs to switch to a cumulative basis (gov.uk). Modern HMRC-recognised payroll software for SMEs carries the starter information straight into the first FPS, so the employer reports the correct starting position rather than defaulting to an emergency code for longer than necessary.

Company benefits and the State Pension

An employee can also be placed on an emergency code when they start receiving company benefits, such as a company car, or when the State Pension comes into payment (gov.uk). These change the amount of untaxed income HMRC has to account for, and until the code is recalculated the emergency marker stays in place (gov.uk).

The timing differs from the new-starter case. Where an emergency code follows company benefits or the State Pension, the employee usually keeps it until the end of the tax year, then moves to a non-emergency code for the following year (gov.uk). The cause is recorded on the coding notice HMRC issues, which sets out how the allowance was reduced (gov.uk).

Second jobs and the 0T or BR code

An employee with more than one job presents a different problem. The Personal Allowance is usually applied to one job, and income from the second is taxed in full under a BR or 0T code (gov.uk). That is not strictly an emergency code, but it has the same effect of removing the allowance, and employees often mistake it for one (gov.uk).

The distinction matters for the employer. A BR code on a second job is usually correct, whereas an emergency W1 or M1 code on a first job is temporary and expected to change (gov.uk). HMRC lets an employee check and, where needed, rebalance allowances across two jobs so the tax-free amount is not wasted (gov.uk).

How payroll applies an emergency code

The W1, M1 and X markers are instructions to the payroll software about the calculation basis, and this is where the practical difference shows up on a payslip (gov.uk).

Cumulative basis

Under a normal, cumulative code the software adds each payment to the running total for the year, works out the tax due on everything earned so far, and deducts the difference between that figure and the tax already taken (gov.uk). This is what allows a refund inside payroll: if too much was deducted earlier in the year, a later payrun gives it back automatically once the correct code is in use (gov.uk).

The cumulative method is the default and the reason an accurate P45 is so useful. It lets the new employer pick up the year-to-date figures and keep the running total unbroken, so the employee's tax settles to the right amount across the year (gov.uk).

Non-cumulative (Week 1 or Month 1) basis

An emergency code runs on a non-cumulative basis. The W1 marker tells the software to treat each week in isolation, and M1 does the same for each month (gov.uk). Each pay period gets one period's worth of allowance, one twelfth of £12,570 in a month, and no account is taken of pay or tax from earlier in the year (gov.uk).

The consequence is that no refund can flow through payroll while an emergency code is in force, because there is no running total to correct against (gov.uk). The X marker is used where the pay dates are irregular and neither a weekly nor a monthly cycle fits cleanly (gov.uk). Whichever marker applies, HMRC's rules also cap any single deduction at 50% of the payment, a safeguard that sits behind every code, emergency or not.

The emergency code and the starter declaration

When a new employee has no P45, the starter checklist asks them to pick a starter declaration, and that choice drives the starting code the employer applies (gov.uk). The three declarations map to codes as follows (gov.uk).

Declaration B produces exactly the emergency, non-cumulative code described above, which is why a new starter who ticks it sees W1 or M1 on the first payslip (gov.uk). An employee who is unsure which declaration applies is handled most safely under declaration B, and the code is corrected once HMRC confirms the position (gov.uk). An HMRC-recognised payroll engine maps each declaration to the right code and calculation basis automatically, so the starting position is defensible even before HMRC issues its own code.

How an emergency tax code gets corrected

An emergency code is designed to be temporary, and two parties move it back to normal: HMRC, which issues the correct code, and the employee, who can supply missing details to speed that up (gov.uk).

What HMRC does

HMRC usually updates the tax code once it has received the employee's details from the new and previous employers, then sends the revised code to both the employee and the employer (gov.uk). Because this relies on Real Time Information reaching HMRC after the first payrun, it can take up to 35 days from the start date for the new code to come through (gov.uk). Payroll software that holds the HMRC Recognised badge submits each FPS automatically and applies the revised code on the next payrun without manual reconfiguration, which is what keeps the 35-day window from stretching further (gov.uk).

Once the correct cumulative code arrives, the employer applies it and the running total is restored, at which point any tax overpaid earlier in the year is repaid through payroll (gov.uk). The employer does not change an emergency code on its own judgement; it waits for HMRC's instruction or a valid P45 (gov.uk).

What the employee can do

An employee does not have to wait passively. They can check the code HMRC holds and the estimate behind it using the Check your Income Tax service for the current year (gov.uk). The same service, reached through a personal tax account, lets them see how the allowance has been allocated and update the details that feed the code (gov.uk).

Where the code still looks wrong after the 35 days, or where it does not reflect a benefit or the State Pension, the employee can update their employment income details online or contact HMRC directly (gov.uk). HMRC also publishes guidance for anyone who thinks their code is wrong, which walks through the evidence to gather first (gov.uk).

Getting money back when too much tax was paid

An emergency code frequently leads to an overpayment, because it withholds the cumulative refunds a normal code would release (gov.uk). How the money comes back depends on when the correct code arrives.

If the cumulative code is applied before the end of the tax year, the refund usually flows through payroll on the next payrun, as the restored running total shows that too much was deducted (gov.uk). If the year ends first, HMRC reconciles the position and, where a refund is due, issues a P800 tax calculation explaining how to claim it (gov.uk). The year-end figures behind this reconciliation are the same ones summarised on the employee's P60, set out in the end-of-year P60 checklist. An employee can confirm the figures for a completed year through the Check your Income Tax service rather than waiting for a letter (gov.uk).

For employers, the key point is that an emergency code rarely leaves the employee permanently worse off. It defers the correct result rather than changing it, and accurate RTI reporting is what brings the two back into line. Accountants managing starters across many clients typically rely on payroll bureau software to flag every employee still sitting on an emergency code so none is overlooked at year end.

Check a tax code before the next payrun

Before running payroll for a new starter, an employer can confirm what a code means and how it will behave using the Moonworkers tax code checker, which decodes the number, the letters and the W1, M1 or X marker against the 2026-27 rules.

Conclusion

An emergency tax code is best understood as a holding position rather than a problem. It applies the standard Personal Allowance one pay period at a time, which protects an employee from a large upfront deduction but also freezes the cumulative refunds a normal code would release. The W1, M1 and X markers are the signal that this is happening, and reading them correctly tells an employer whether a code is temporary and about to change or a settled instruction such as a second-job BR code.

The wider shift to Real Time Information has made emergency codes far less consequential than they once were. Where year-end was previously the only moment tax settled, accurate FPS reporting now corrects most codes within a few weeks, and refunds follow automatically once a cumulative code is in use. The direction of travel is towards faster reconciliation still, as more payroll data reaches HMRC in real time and the gap between starting a job and holding the right code continues to narrow.

Frequently asked questions

What does W1 or M1 after a tax code mean on a payslip?

W1 and M1 mark an emergency tax code applied on a non-cumulative basis. W1 means the tax is worked out on that week's pay alone, and M1 does the same for a month, with one period's share of the Personal Allowance and no account taken of earlier pay or tax in the year (gov.uk). Some payroll systems show NONCUM instead of W1 or M1 (gov.uk).

How long does an emergency tax code last?

For a new starter, an emergency code is usually temporary and is replaced once HMRC has the details it needs, which can take up to 35 days from the start date (gov.uk). Where the code follows company benefits or the State Pension, the employee usually keeps it until the end of the tax year and then moves to a normal code (gov.uk).

Will an employee get back tax overpaid on an emergency code?

Yes. If the correct cumulative code is applied before the tax year ends, the refund normally comes through payroll on the next payrun (gov.uk). If the year has already ended, HMRC reconciles the position and issues a P800 calculation setting out any refund due (gov.uk).

How can an employee get off an emergency tax code faster?

Giving the new employer a P45 from the previous job is the quickest route, because it supplies the year-to-date figures needed for a cumulative code (gov.uk). Where no P45 exists, the employee can check and update the details HMRC holds through the Check your Income Tax service and a personal tax account (gov.uk).