Your tax code tells your employer how much Income Tax to deduct. HMRC issues new codes each April but errors are common — a wrong code means overpaying or underpaying for an entire year. The most common code is 1257L — the digits represent your £12,570 Personal Allowance, the L means standard allowance.
Other key codes: BR (all at 20%, no allowance — typical for second jobs), D0 (all at 40%), K (negative allowance), W1/M1 (emergency week/month 1 basis).
Your tax code is one of the most important pieces of information on your payslip — yet most people never check it. An incorrect tax code can cost you hundreds or even thousands of pounds in overpaid or underpaid tax. This tool decodes any UK tax code and tells you whether it looks right for your circumstances.
A tax code is usually a number followed by a letter. The number represents your tax-free allowance divided by 10. For example, 1257 means a tax-free allowance of £12,570. The letter indicates your tax situation: L means you get the standard allowance, M means you have received Marriage Allowance from your partner, N means you have transferred Marriage Allowance to your partner, T means your tax code has been adjusted by HMRC, and K means your deductions exceed your allowances.
Emergency tax codes like BR (basic rate on all income), D0 (40% on all income), and 0T (no personal allowance) are temporary but can persist if HMRC does not receive correct information. The W1 and M1 suffixes mean tax is calculated on a "week 1/month 1" basis — essentially emergency tax that does not account for your cumulative income.
Tax codes go wrong when you change jobs without a P45, have multiple employments, receive taxable benefits, claim Marriage Allowance, or have underpaid tax in a previous year. HMRC adjusts your code to collect underpaid tax through your salary — look for a reduction in your tax-free amount. If your code changes unexpectedly, call HMRC on 0300 200 3300 or check your Personal Tax Account online.
| Code | Meaning | Who It Applies To |
|---|---|---|
| 1257L | Standard Personal Allowance (£12,570) | Most employees with one job |
| BR | Basic Rate (20% on all income) | Second jobs, pensions |
| D0 | Higher Rate (40% on all income) | Second jobs for higher earners |
| NT | No Tax | Non-residents, special cases |
| K1257 | Negative allowance — tax owed | Those with untaxed income exceeding allowance |
| 0T | No Personal Allowance | When allowance is used elsewhere |
| M | Marriage Allowance recipient | Received 10% of partner's allowance |
| N | Marriage Allowance transferrer | Gave 10% of allowance to partner |
| S | Scottish tax rates | Scottish taxpayers |
| C | Welsh tax rates | Welsh taxpayers |
Your tax code can be wrong for several reasons. Here are the most common:
What to do: Check your latest tax code notice (P2) from HMRC. If it looks wrong, call HMRC on 0300 200 3300 or use your personal tax account.
The number is your tax-free Personal Allowance divided by 10. For example, 1257 means £12,570 tax-free. If you have deductions (company benefits, unpaid tax), these are subtracted from your allowance, giving a lower number.
Use our calculator above to compare your code against your salary. If the result says "Check needed", you may be underpaying or overpaying. Also watch for: emergency codes lasting more than a month, a K code when you don't have untaxed income, or a BR code on your only job.
Emergency codes (ending W1 or M1, or starting with X) mean your employer is taxing you on a "week 1/month 1" basis without your full tax history. You'll usually pay more tax initially, but it corrects once HMRC sends your proper code. It should not last more than 1-2 months.
All calculations are verified against official HMRC thresholds and rates for the 2026/27 tax year. Figures are updated within 24 hours of any HMRC announcement. Calculations are for guidance only — consult a qualified accountant for personalised advice.