How W1 works mechanically
A normal cumulative code (e.g. 1257L) calculates tax based on year-to-date earnings. Each pay period, payroll asks: "what's the correct year-to-date tax position?" and deducts the difference.
W1 changes that to: "what's the correct tax just for this week's earnings, assuming 1/52 of the annual allowance and 1/52 of each tax band?"
For 1257L W1 in 2026/27: - Weekly personal allowance: £12,570 / 52 = £241.73 - Weekly basic-rate band: £37,700 / 52 = £725 (tax-free + tax-free + first £725 at 20%) - Weekly tax (first £241.73): £0 (allowance) - Weekly tax (next £725): 20% = up to £145 - Weekly tax (above £725 + £241.73 = £966.73): 40%
So someone earning £25,000/year (= £481/week) on 1257L W1 pays: - Tax-free £241.73 - Taxable £239.27 × 20% = £47.85/week - Annualised: £2,488/year
This matches the cumulative 1257L outcome for steady weekly pay — the W1 calculation is just the same maths applied weekly without YTD catch-up.
Worked example — variable pay on W1
Same person, but with one big-overtime week:
| Week | Gross | W1 tax | Cumulative 1257L tax (catching up) |
|---|---|---|---|
| Normal week | £481 | £48 | £48 (matches) |
| Overtime week | £900 | (£241 free + £659 × 20%) = £132 | Higher in that week if cumulative thinks YTD has jumped into higher rate — but typically same |
| Following week (back to normal) | £481 | £48 | Cumulative may credit back overpaid |
W1 doesn't smooth out variable weeks. A big overtime week pays full tax on the extra; a quiet week gets the full weekly allowance. Over the year these usually balance out.
When W1 ends
W1 is temporary. It ends when:
- HMRC issues a cumulative tax code — typically after receiving your previous employment data via P45 or new-starter checklist
- The new tax year starts (6 April) — all codes effectively reset; you may continue on W1 if circumstances haven't been clarified, or get a cumulative code
For most workers, W1 ends within 2-6 pay weeks of starting a new job.
What to do if W1 persists
If you've been on W1 for >8 weeks and no cumulative code has been issued:
- Log into HMRC Personal Tax Account at gov.uk/personal-tax-account
- Check your current employment is listed correctly
- Confirm previous employer's data has been submitted
- Update any missing fields (start date, employment statement)
- Wait 2-4 weeks for HMRC to issue a corrected code
If self-service doesn't resolve, call HMRC on 0300 200 3300.
W1, M1, X — emergency code suffixes compared
| Suffix | Pay frequency | Allowance per period |
|---|---|---|
| W1 | Weekly | 1/52 (~£241.73) |
| M1 | Monthly | 1/12 (~£1,047.50) |
| X | Non-cumulative, frequency-agnostic | Whatever proportion fits |
The mechanism is identical; the difference is just how the allowance is sliced. Read about M1 → and X →.
W1 + your payslip
Your payslip will show your code as something like: - 1257L W1 — standard allowance, weekly emergency - 1257L X — same thing using the generic non-cumulative marker
The tax deduction line will look broadly similar to cumulative 1257L for steady weekly pay. Variation from week to week is normal under W1.
Common worries (and reassurances)
"I'm being overtaxed!" — Not necessarily. Compare your weekly tax to what you'd pay on cumulative 1257L. Usually within £5/week of each other.
"I'll never get the overpayment back!" — You will. Either HMRC issues cumulative and your next pay run catches up, OR HMRC's annual P800 reconciliation (June-October following tax year) refunds via PAYE or cheque.
"My employer keeps me on W1 deliberately!" — Very unlikely. W1 is HMRC-issued; your employer applies whatever HMRC tells them. To investigate, log into Personal Tax Account first; if HMRC's records show cumulative but your employer is still using W1, then it's a payroll bug — flag it.
Scotland and W1
Scottish taxpayers see S1257L W1 or similar. Same mechanism; Scottish bands applied weekly: - 19% starter (£0–£54/wk) - 20% basic (£54–£287/wk) - 21% intermediate (£287–£598/wk) - 42% higher (£598–£1,201/wk) - 45% advanced (£1,201–£2,406/wk) - 48% top (above £2,406/wk)
(All approximate, divided by 52.)
Quick verification
If you're on W1 and want to sanity-check the deduction:
- Take your weekly gross
- Subtract £241.73 (the weekly personal allowance for 1257L W1)
- The remainder × 20% (if under £725 in that week) = expected tax
If actual matches expected ±£5, the calculation is correct. If actual is materially higher, query payroll.
Practical checklist
- Check your payslip code ends in "W1" (or "X")
- Compare the weekly tax to expected cumulative
- If under 4 pay periods: wait, usually resolves automatically
- If over 6 pay periods: log into Personal Tax Account
- If overpayment is substantial: the emergency tax refund guide → covers next steps
In short
W1 means weekly emergency basis — each pay period calculated standalone with 1/52 of your allowance. Common after a new job; usually self-corrects within 2-6 weeks. For variable-pay workers the deduction varies week to week. See emergency tax → for the broader picture and tax codes hub → for the full reference.