An emergency tax code is a temporary code that ends in W1 (weekly), M1 (monthly) or X. Instead of taxing your income cumulatively across the year, it taxes each pay period on its own, as if that single payday repeats every period. Because it ignores what you have earned and paid so far, it usually overtaxes you. HMRC issues it when your employer does not yet have enough detail to run a normal tax code.
How is Week 1 / Month 1 different from a normal code?
A normal code is cumulative: PAYE looks at your total pay since 6 April and total tax paid, then balances the two each payday, so over- and under-payments self-correct. A W1/M1 code is non-cumulative: it forgets every previous period and gives only that period's share of your allowance. You may see NONCUM on the payslip. If your earlier pay was low or nil, the code denies you the unused allowance you had built up, so the deduction is too high.
What puts me on an emergency code?
Common triggers, per HMRC:
- Starting a new job without a P45 from your last one (see what to do if you lost your P45).
- Starting work after a spell self-employed or claiming benefits.
- Beginning to receive company benefits, such as a company car or private medical cover.
- Starting to draw the State Pension or a workplace pension partway through the year.
How do I get off it and reclaim overpaid tax?
Give your new employer the details on the HMRC starter checklist so they can report you correctly. HMRC then issues a normal cumulative code, usually within 35 days of your start date. Once the code is corrected, PAYE recalculates your year to date and repays any overpaid tax through your next payslip automatically, no claim needed. If the tax year ends before it self-corrects, you can claim the refund directly. Check your personal tax account to confirm the live code.
Primary sources
- Emergency tax codes — gov.uk — What W1, M1 and X mean and how the code is corrected
- Claim a tax refund — gov.uk — Reclaiming overpaid tax from an emergency code
- The Income Tax (Pay As You Earn) Regulations 2003 — legislation.gov.uk — Statutory basis for cumulative and non-cumulative PAYE
- Starter checklist for PAYE — gov.uk — Form that gives a new employer the details to avoid emergency tax
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