UK Tax Codes · 2026/27
W1 / M1 Tax Code Meaning — Emergency / Week 1 / Month 1 (UK 2026/27)
Who gets the W1 / M1 (emergency) tax code?
W1/M1 most commonly applies to (1) new employees who haven't given their P45 to a new employer, (2) people returning from extended leave or unemployment without recent UK income history, and (3) people whose tax position has changed mid-year and HMRC is treating each subsequent pay period in isolation while it recalculates.
How W1 / M1 (emergency) affects your pay
W1/M1 treats each pay period as a self-contained unit, using 1/52 (weekly) or 1/12 (monthly) of your personal allowance. The result is usually correct in the short term but can lead to incorrect totals if your earnings vary across periods. The good news: at year-end reconciliation, HMRC adjusts and refunds any overpayment.
When to check this code
Check W1/M1 whenever you start a new job — provide your P45 immediately to resolve it. Also check after any payroll change. W1/M1 is usually a temporary code that resolves within 1-2 pay periods once HMRC has full information.
What to do if it's wrong
Provide your P45 to your new employer if you have one. If you don't, ask your employer for the P46 starter checklist — completing this gives HMRC enough information to issue a normal cumulative code. Without P45 or starter checklist, W1/M1 can persist for the whole tax year, with year-end reconciliation as the fix.
Example calculation
On £4,000/month with 1257L W1: each month treated independently. £4,000 − (£12,570/12) = £4,000 − £1,047.50 = £2,952.50 taxed at 20% = £590.50/month. Annual: ~£7,086. Same as cumulative 1257L would produce in this stable-income case. The difference shows up if monthly income varies — W1/M1 doesn't smooth the calculation.
Recruiter pro tip
W1/M1 is fine for most new starters once their P45 lands. The risk: if you have a high-earning month early in the tax year then drop to lower income, W1/M1 doesn't smooth — you'll overpay each high month and underpay each low month. The year-end reconciliation fixes it but you've been giving HMRC an interest-free loan for months. Get cumulative coding established quickly via P45/P46 to avoid this.
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