That combination of numbers and letters on your payslip — 1257L, BR, D0, K497 — tells your employer exactly how much tax-free income you're entitled to. Most people glance at it once and never think about it again. But if HMRC gets your code wrong, you could be overpaying or underpaying tax for months without realising.
How tax codes work
The number in your tax code represents your tax-free allowance divided by 10. So a code of 1257L means you have a tax-free allowance of £12,570 — the standard personal allowance. Your employer uses this to work out how much of your pay is taxable each pay period.
The letter tells your employer which rate of tax to apply and under what circumstances. The most common letters are:
- L — standard personal allowance applies
- M — you've received a Marriage Allowance transfer from your spouse
- N — you've transferred part of your allowance to your spouse
- T — HMRC needs to review your code (often used when they want more information)
- 0T — no personal allowance (you've used it up elsewhere or HMRC has an issue)
- BR — all income taxed at basic rate (20%), usually for a second job
- D0 — all income taxed at higher rate (40%)
- D1 — all income taxed at additional rate (45%)
The K code
A K code means your deductions (company car benefit, unpaid tax from previous years, state pension adjustments) exceed your personal allowance. Instead of having tax-free pay, tax is effectively added to your income. For example, K497 means £4,970 is added to your taxable income. It looks alarming, but it's HMRC's way of collecting tax you owe without sending you a separate bill.
Scottish and Welsh tax codes
If you live in Scotland, your code starts with S (e.g., S1257L). Scottish taxpayers have different income tax rates set by the Scottish Parliament. Welsh codes start with C (e.g., C1257L), though Welsh rates have so far matched the English ones.
Why your code might be wrong
Tax code errors are surprisingly common. They often happen when:
- You change jobs and HMRC doesn't update your records promptly
- You start or stop receiving a taxable benefit (company car, medical insurance)
- You have multiple jobs and the allowances aren't split correctly
- HMRC is collecting underpaid tax from a previous year through your code
- Your estimated income crosses a threshold mid-year
If your code looks wrong — or if you've suddenly started paying noticeably more or less tax — check it through your Personal Tax Account on GOV.UK. You can update your employment details, report benefits, and see HMRC's calculation of your code.
Emergency tax codes
When you start a new job without a P45 from your previous employer, you might be put on an emergency tax code. This could be 1257L W1, 1257L M1, or 1257L X. The W1/M1/X suffix means your tax is calculated on a non-cumulative basis — each pay period is treated in isolation, without accounting for previous months' earnings and tax. This often results in overpayment, which should be corrected once HMRC issues the right code.
If you're still on an emergency code after a couple of months, chase it up. Contact HMRC or check your Personal Tax Account to make sure they have the right information about your previous employment.
What to do if you've overpaid
HMRC runs an annual reconciliation process (P800) after each tax year. If you've overpaid, they'll either adjust your tax code for the following year or send you a refund. You can claim a refund through your Personal Tax Account or by phoning HMRC on 0300 200 3300. Refunds can go back four years, so if you think you've had the wrong code for a while, it's worth investigating.