Tax Code W1 or M1 Explained

What Does W1 or M1 Mean?

W1 (weekly) or M1 (monthly) means you're on an emergency tax basis. Your employer calculates tax on each pay period independently rather than cumulatively. This often happens when starting a new job without a P45. You'll usually overpay tax and get a refund once HMRC issues your correct code.

Is My Tax Code Correct?

If you think your tax code is wrong, you can check it on your payslip, through your HMRC online account, or by calling HMRC on 0300 200 3300. A wrong tax code means you're paying too much or too little tax.

All Common Tax Codes

What W1-M1 Means

W1 (Week 1) or M1 (Month 1) are emergency tax codes, sometimes shown as '1257L W1' or '1257L M1' or just 'X'. They mean your tax is calculated on a non-cumulative basis — each pay period is treated independently rather than cumulatively across the year. This can result in overpaying or underpaying tax.

When This Code Is Used

Emergency codes are applied when you start a new job without a P45 or starter checklist, when HMRC needs to update your code but has not yet done so, or during the tax year when your circumstances change. Your employer cannot change an emergency code — only HMRC can issue a cumulative code to replace it.

Impact on Your Pay

On an emergency code, you may not receive your full Personal Allowance spread across the year. For example, starting a job in September on M1 means you only receive 1/12th of your allowance per month, not the accumulated allowance for the months you were not employed. Once HMRC issues a cumulative code, overpaid tax is refunded through your payslip.

Learn more: Full guide to UK tax codes