Understanding W1 or M1 Tax Codes

If your employee’s tax code has ‘W1’ or ‘M1’ at the end

W1 (week 1) and M1 (month 1) are emergency tax codes and appear at the end of an employee’s tax code, eg ‘577L W1’ or ‘577L M1’. Calculate your employee’s tax only on what they are paid in the current pay period, not the whole year.

Updating for the new tax year

HM Revenue and Customs (HMRC) will tell you between January and March about any new tax codes to use for employees in the new tax year. This starts on 6 April.

If an employee’s tax code isn’t changing, HMRC won’t contact you and you should carry forward the employee’s tax code to the new tax year.

If your employee’s tax code ends with ‘M1’ or ‘W1’ (‘month 1’ or ‘week 1’), don’t carry this part of the code into the new tax year.

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