Last reviewed January 2023

A tax code is a string of numbers and letters that tells your employer how your earnings should be taxed. You get one for each Pay As You Earn (PAYE) job you have, and it’s important to keep track of them. Any time your tax code changes, HMRC will alter the Income Tax you’re paying each year.

M1 is a kind of “emergency tax code”, meaning it’s only supposed to be temporary until HMRC has enough information to issue you the right code. The big thing to know about the M1 tax code is that it’s “non-cumulative”, so it doesn’t take into account any tax you’ve already paid in the tax year.