How M1 works mechanically
Cumulative tax code: each pay period, payroll asks "what's the correct year-to-date tax?" and reconciles.
M1: each month is standalone. The calculation:
- Monthly personal allowance: £12,570 / 12 = £1,047.50
- Monthly basic-rate band: £37,700 / 12 = £3,141.67 of band capacity
- Monthly higher-rate band: starts at £4,189.17 monthly (allowance + basic band)
- Tax is calculated on just this month's gross, regardless of prior months
For 1257L M1 on £50,000/year (= £4,167/month): - Tax-free: £1,047.50 - Basic rate (£3,141.67 × 20%): £628.33 - Higher rate (remaining £-22 — actually below higher threshold this month): £0 - Total monthly tax: ~£604
This matches cumulative 1257L for steady monthly pay.
Worked example — steady pay on M1
Same person, £50,000/year on M1, all 12 months identical:
- Monthly gross: £4,167
- Monthly tax: ~£604 (per above)
- Monthly NI: ~£250
- Monthly net: ~£3,313
Cumulative 1257L would produce the same result. M1 isn't penalising you for steady pay.
Worked example — bonus month on M1
Same £50,000 base + £5,000 bonus paid in October:
October pay: £4,167 + £5,000 = £9,167 gross
M1 calculation for October: - Tax-free: £1,047.50 (single month allowance, not catching up) - Basic rate (£3,141.67 × 20%): £628.33 - Higher rate (£9,167 − £4,189.17 = £4,977.83 × 40%): £1,991.13 - Total monthly tax: £2,619
October net: £9,167 − £2,619 − higher NI = roughly £6,300
Cumulative 1257L would tax the same bonus month differently — checking year-to-date earnings and potentially recovering some of the higher-rate hit if previous months had basic-rate band capacity unused. M1 doesn't do that.
For bonus months, M1 typically over-deducts slightly. When the cumulative code resumes (or year-end reconciliation occurs), the overpayment refunds.
When M1 ends
M1 is temporary. Resolution paths:
- HMRC issues a cumulative code to your employer after building your employment record (typical timeline: 4-8 weeks after job start)
- The new tax year starts (6 April) and HMRC re-establishes a code for the new year
- You actively update your HMRC Personal Tax Account with missing details
For most monthly-paid workers, M1 lasts 1-3 months.
What to do if M1 persists past 3 months
- Log into HMRC Personal Tax Account at gov.uk/personal-tax-account
- Check current employment is listed correctly
- Verify previous employer's submission (P45 or new-starter checklist)
- Update any missing data
- Wait 2-4 weeks for HMRC to issue a corrected code
If self-service doesn't fix it, call HMRC on 0300 200 3300.
M1 + bonus or one-off pay
If you know a bonus is coming and you're still on M1, options:
Option A — accept the slight over-deduction. Bonus is taxed in full in the month it's paid; M1 might deduct a bit more than cumulative. You'll get it back when M1 ends.
Option B — request cumulative code first. Contact HMRC and ask them to expedite the cumulative code. Often successful if you're 1-2 months in.
Option C — pension salary sacrifice on the bonus. Reduces the bonus before tax is calculated, applies in any code. Worth considering for higher-rate earners.
M1 vs W1 vs X
| Code | Frequency | Allowance/period | Common use |
|---|---|---|---|
| M1 | Monthly | £1,047.50 (1/12) | New job, monthly-paid |
| W1 | Weekly | £241.73 (1/52) | New job, weekly-paid |
| X | Either | Pro-rata | Generic non-cumulative |
Same mechanism, different pay frequency. Read about W1 → and X →.
M1 + multiple income sources
If your main job is on M1 and you have a second income source:
- Your main job applies the £1,047.50 monthly allowance
- Your secondary source likely has BR/D0/0T (no allowance)
- Total income may push you into higher rates that M1 doesn't yet know about
When HMRC issues a cumulative main code, it accounts for the secondary source and adjusts. Pre-cumulative, you might slightly underpay; the catch-up happens automatically.
Scotland and M1
Scottish taxpayers see S1257L M1 or similar. Same mechanism with Scottish bands: - 19% starter rate (£0–£236/mo) - 20% basic (£236–£1,243/mo) - 21% intermediate (£1,243–£2,591/mo) - 42% higher (£2,591–£5,203/mo) - 45% advanced (£5,203–£10,428/mo) - 48% top (above £10,428/mo)
(All approximate monthly equivalents of annual figures, divided by 12.)
Quick verification
To sanity-check your M1 deduction:
- Take your monthly gross
- Subtract £1,047.50 (monthly allowance for 1257L M1)
- If under £3,141.67 remaining: × 20% = expected tax
- If above: first £3,141.67 × 20% + remainder × 40%
If actual deduction matches expected (within £5), the calculation is correct. If materially higher, query payroll.
Common worries (and reassurances)
"My take-home is lower than expected!" — Compare against the cumulative-1257L expected take-home. If similar, M1 is fine. If materially lower, there may be another issue (wrong tax code base, missing P45 info).
"Will I lose money permanently?" — No. Every overpayment under M1 is refunded either when cumulative resumes (typical) or at year-end via P800 (worst case).
"What if M1 lasts until tax year end?" — HMRC's P800 reconciliation auto-corrects after 5 April. You'll receive a tax calculation letter (June-October) explaining any refund or shortfall.
Practical checklist
- Check your payslip code ends in "M1" (or "X" for the generic version)
- Compare monthly tax to cumulative 1257L (should be similar for steady pay)
- Wait 1-2 pay periods for M1 to typically resolve automatically
- If M1 persists past 3 months, log into Personal Tax Account
- Substantial overpayment: see emergency tax refund guide →
In short
M1 means monthly emergency basis — each month calculated standalone with 1/12 of your allowance. Common after starting a new job; usually self-corrects within 1-2 months. For steady-pay workers, tax is roughly the same as cumulative; for variable months (bonus, overtime), short-term over-deduction may occur and is refunded when the cumulative code resumes. See emergency tax → and tax codes hub →.