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HMRC changed my tax code because of savings interest — and got it wrong
7 min read
Your tax code is not random. Every digit and every letter has a specific meaning that tells your employer exactly how to calculate your tax. Here's a complete guide to every code you might encounter.
The Number: Your Tax-Free Allowance
The number in most tax codes (e.g., "1257" in 1257L) represents your annual tax-free Personal Allowance divided by 10. So 1257 = £12,570 personal allowance. Your employer uses this to calculate what portion of your pay is taxable each month.
The Letters: What They Mean
| Letter | Meaning | Impact on Tax |
|---|---|---|
| L | Standard personal allowance (most common) | Normal 20%/40% rates apply above your allowance |
| M | Marriage Allowance received (extra 10% of partner's allowance) | Slightly lower tax — you've received £1,260 of partner's allowance |
| N | Marriage Allowance transferred to partner | Slightly higher tax — you've given away £1,260 of your allowance |
| T | HMRC is reviewing your tax affairs | Temporary — should resolve once reviewed |
| BR | All income taxed at Basic Rate (20%) | No personal allowance — used for second jobs |
| D0 | All income taxed at Higher Rate (40%) | Used if second job pushes into 40% band |
| D1 | All income taxed at Additional Rate (45%) | Very high earners with multiple income sources |
| 0T | No personal allowance — emergency code | Taxed at 20% from the first pound (worse than BR if above basic rate threshold) |
| K | Negative allowance — you have deductions greater than your allowance | Tax is added to your gross pay before calculating — you pay more than normal |
| NT | No tax | Pay no tax — usually for non-UK residents or specific situations |
| S prefix | Scottish rates apply | e.g., S1257L means you pay Scottish income tax rates |
| C prefix | Welsh rates apply | e.g., C1257L — Welsh Rate of Income Tax applies |
| W1/M1 | Emergency "week 1/month 1" basis — non-cumulative | Tax calculated on each pay period in isolation rather than cumulatively. Can cause over or underpayment. |
The Most Common Wrong Codes
- BR or 0T when you only have one job — this means you're paying tax from pound one with no personal allowance. Must be challenged.
- Lower number than 1257 without reason — something is reducing your allowance that shouldn't be.
- W1 or M1 applied long-term — these should be temporary emergency codes, not permanent.
Frequently Asked Questions
Can I just ask HMRC to change my code?
Yes — log into your Personal Tax Account at gov.uk and update the information HMRC holds about you. Alternatively, call 0300 200 3300 with your National Insurance number ready.
How quickly does a new code take effect?
HMRC sends the updated code electronically to your employer, usually within 2–5 working days. Your employer applies it at the next payroll run.
Check your specific code using our Tax Code Checker.