Emergency Tax Code Explained: W1, M1 and X

An emergency tax code is a temporary code that taxes each payday on its own, without taking account of what you have earned so far in the year. You can spot one by the letters W1, M1 or X after the numbers, and it usually appears when HMRC does not yet have the full picture of your income, most often when you start a new job without a P45.

In this insight we explain when you get an emergency code, why it can leave you paying too much tax, how to get onto the correct code, and how to reclaim anything you have overpaid.

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Key Takeaways

• An emergency tax code is non-cumulative, meaning each pay period is taxed in isolation rather than across the whole year.
• You usually get one when you start a new job without a P45, take your first job, or start a pension, and HMRC lacks your full details.
• Emergency codes can cause you to overpay tax, especially if your income varies or you have unused allowance earlier in the year.
• Once HMRC has your details, your code is corrected and any overpaid tax is normally refunded through your pay or after the tax year.

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What Is an Emergency Tax Code?

An emergency tax code tells your employer to work out your tax based only on the current pay period, ignoring your earnings and tax paid earlier in the year. This is why it is described as non-cumulative.

You will recognise it by W1 (week 1), M1 (month 1) or X after the numbers, for example 1257L W1. The numbers may still reflect the standard Personal Allowance, but the W1, M1 or X marker changes how it is applied. Instead of smoothing your allowance and tax across the year, each payslip is treated as if it were the first of the year.

For a fuller picture of how codes are built, see our guide to tax codes explained.

When Do You Get One?

Emergency codes are common at transition points, when your new employer or pension provider does not yet have complete information from HMRC. You might be put on one if you:

• Start a new job and cannot give your employer a P45 from your last one.
• Start your very first job.
• Begin receiving a workplace or private pension.
• Move from self-employment into employment.
• Have started receiving company benefits and HMRC is catching up.

In many of these cases your employer uses a starter checklist to gather basic details, but until HMRC confirms your correct code, the emergency version fills the gap.

Why an Emergency Code Can Cost You

Because a non-cumulative code ignores your earlier earnings, it does not give you the benefit of any unused Personal Allowance from the start of the year. If you began a job partway through the year, or your pay is uneven, you can end up paying more tax than you should.

For example, if you were not working for the first few months of the tax year, you would normally have unused allowance to set against later pay. A cumulative code takes that into account; an emergency code does not, so more tax comes off until the code is corrected.

You can estimate what your take-home should look like on the correct code using our income tax calculator, which helps you see whether an emergency code is costing you.

How to Get Onto the Right Code

The key is making sure HMRC has your complete, up-to-date details. To move off an emergency code:

• Hand over your P45 to your new employer if you have one from a previous job.
• Complete the starter checklist your employer gives you if you do not have a P45, answering the questions accurately.
• Check your personal tax account on GOV.UK to confirm HMRC has the right employment details.
• Contact HMRC if your code does not update after a pay period or two, so they can issue the correct one.

Once HMRC sends your employer the right code, your pay is recalculated. If your code changed for another reason, our guide to why your tax code has changed may help.

Reclaiming Overpaid Tax

If an emergency code caused you to overpay, you usually get the money back automatically. When your correct cumulative code is applied, your next payslip often includes a refund as the calculation catches up across the year.

If the overpayment is not resolved through your pay, HMRC typically reconciles it after the tax year ends and issues a refund. You can also check and prompt this through your personal tax account. There is no need to pay anyone to claim a standard PAYE refund on your behalf.

Frequently asked questions

How long does an emergency tax code last?

It lasts until HMRC has enough information to issue your correct code, which can be as quick as one or two pay periods once your details are in. If it drags on, contact HMRC or check your personal tax account to speed things up.

Will I get my overpaid tax back automatically?

Usually, yes. Once your correct cumulative code is applied, your pay is recalculated and any overpayment is often refunded through your salary. If not, HMRC normally sorts it out after the tax year and refunds you directly.

Does an emergency code mean I have done something wrong?

No. It simply means HMRC does not yet have your full details, which is completely normal when starting a job or a pension. It is a temporary measure, not a penalty.

What is the difference between W1, M1 and X?

They mean the same thing but relate to how often you are paid. W1 is used for weekly pay, M1 for monthly pay, and X is a general marker. All three signal a non-cumulative code where each pay period is taxed on its own.

General information only, not financial advice. If you think your tax code is wrong, contact HMRC.