W1 and M1 Tax Codes Explained

If your tax code ends in W1, M1 or X, you are on an emergency, non-cumulative tax basis. It is a temporary code that often means you are paying too much tax. This guide explains what W1 and M1 mean and how to get back onto the right code.

What W1 and M1 mean

W1 stands for week 1 and M1 for month 1. They tell your employer to work out your tax on each pay period on its own, rather than across the whole year so far. X is used where the pay frequency is not weekly or monthly. These are emergency codes, used when HMRC does not yet have your full pay and tax history.

Why a W1 or M1 code can cost you

A normal cumulative code spreads your Personal Allowance evenly and corrects any over or under payment as the year goes on. A W1 or M1 code cannot do that, so if you have unused allowance from earlier in the year, you do not get it back automatically and you often overpay. It commonly appears when you start a new job without a P45.

How to get back on the right code

Once HMRC has your full details, usually from your employer or your P45, they normally issue a correct cumulative code such as 1257L, and any overpaid tax is refunded through your pay or after the tax year ends. You can speed this up by giving your employer your P45 or checking your details in your Personal Tax Account. See also our emergency tax code guide.

Frequently asked questions

What does W1 or M1 mean on my tax code?

They mean week 1 or month 1, an emergency non-cumulative basis where tax is worked out on each pay period alone rather than across the year.

Why am I on a W1 or M1 code?

Usually because HMRC does not yet have your full pay history, often when you start a new job without handing in a P45.

Does a W1 or M1 code mean I am paying too much tax?

Often yes, because the code cannot give back unused allowance from earlier in the year, so you can overpay until it is corrected.

How do I get off an emergency W1 or M1 code?

Give your employer your P45 or update your details with HMRC. They then issue a normal cumulative code and refund any overpaid tax.