How M codes work
A standard tax code is 1257L (£12,570 allowance). An M code adds 10% of your spouse's allowance:
- £12,570 + £1,260 (10% of partner's £12,570) = £13,830 total allowance
- Tax code: 1383M
The Income Tax savings:
- £1,260 × 20% (basic rate) = £252 per year
This is the maximum benefit — anyone earning between £12,570 and £50,270 receives the full £252 saving. Below £12,570 you wouldn't pay any tax anyway; above £50,270 you stop qualifying.
Worked example — £35,000 salary with M code
Without Marriage Allowance (1257L): - Taxable: £35,000 - £12,570 = £22,430 - Income Tax: £22,430 × 20% = £4,486
With Marriage Allowance (1383M): - Taxable: £35,000 - £13,830 = £21,170 - Income Tax: £21,170 × 20% = £4,234
Saving: £252 per year.
For a couple where one partner earns £35,000 and the other earns £0–£12,570, this is essentially free money for being married. Worth checking if you haven't applied — backdating up to 4 years adds up.
Qualifying for Marriage Allowance
Both partners must meet criteria:
The recipient (M code): - Must be married or in a civil partnership - Annual income: up to £50,270 (basic rate) - NOT additional or higher-rate taxpayer
The giver (N code, the lower-earning partner): - Must be married or in a civil partnership to you - Annual income: under £12,570 (or unemployed) - Must be the partner applying on your behalf
If either income changes — recipient earns over £50,270, or giver earns over £12,570 — eligibility ends and HMRC reverts both codes.
How to apply
The lower earner applies on behalf of the couple:
- Go to gov.uk/apply-marriage-allowance
- Sign in to the Personal Tax Account or set one up
- Confirm both partners' details
- Application processed in roughly 4 weeks
- HMRC issues new codes: M for recipient, N for giver
The application is free. Multiple "Marriage Allowance" advert services online charge a fee for what HMRC does for free — avoid them.
Backdating
You can backdate Marriage Allowance up to 4 tax years (currently meaning back to 2022/23). For each backdated year:
- The recipient gets a refund of up to £252 (paid through PAYE or as a lump sum)
- The giver's allowance is reduced retrospectively (usually no tax impact if they earned under £12,570)
A successful 4-year backdated claim is worth up to £1,008 in refunded tax. Worth doing if you qualified in those years but didn't apply at the time.
When M codes change
Marriage Allowance ends and the M code reverts to L when:
- Recipient becomes a higher-rate taxpayer (income exceeds £50,270)
- Giver's income exceeds £12,570 (no longer eligible to give the allowance)
- Divorce or dissolution of civil partnership
- Either partner cancels the arrangement through their Personal Tax Account
HMRC tracks both partners' incomes and reverses the transfer automatically when triggers occur.
M code in Scotland and Wales
Scottish and Welsh taxpayers receive S1383M (Scottish) or C1383M (Welsh) — same allowance increase, Scottish/Welsh band structure applied to the income.
Note: the £50,270 income limit for receiving Marriage Allowance is a UK-wide rule — in Scotland, the higher-rate threshold is lower (£43,663), but Marriage Allowance eligibility is still tested against £50,270.
Common M-code questions
Can I share more than 10%? No — Marriage Allowance is fixed at 10% of one partner's allowance (£1,260). There's no half measure or full transfer.
Can both partners benefit? No — only one partner receives, the other gives. The benefit is mutual in the household but only one M code applies.
Does cohabiting count? No — Marriage Allowance requires a legal marriage or civil partnership. Cohabiting partners (even with children together) don't qualify.
Does Civil Partnership count? Yes — full equivalent of marriage for this purpose.
In short
Tax code M means your annual personal allowance is £1,260 higher than standard because you've received 10% of your spouse's allowance through UK Marriage Allowance. Saves up to £252 a year in Income Tax. To qualify: you must be a basic-rate taxpayer (earnings below £50,270) and your partner must earn under £12,570. Apply free at gov.uk. For the full tax-code reference, see the tax codes hub →. To check eligibility, use the Marriage Allowance checker →.