UK tax code decoder.
Personal Allowance from code
—
What the number in your tax code really means
Your tax code is HMRC's shorthand to your employer's payroll. The numbers carry one job only: take the digits, multiply by ten, and you get the tax-free Personal Allowance your employer will give you across the year before any income tax is deducted under PAYE. A code of 1257L means £12,570 of pay can pass through untaxed in the 2026/27 year, which is the standard Personal Allowance for almost everyone earning under £100,000. Spread across twelve monthly payslips, payroll quietly hands you roughly £1,047 of allowance each month, and taxes the rest at your marginal band.
This decoder reads the digits and the trailing letter and tells you the allowance the code implies. It is deliberately literal: it reports what the code instructs payroll to do, not whether HMRC has set the right code for you. That distinction matters, because a wrong code is one of the most common reasons people overpay or underpay tax during the year and only find out after the tax year closes.
Reading the letters after the number
The suffix changes the meaning. An L is the plain-vanilla code with a full Personal Allowance. An M means you have received the Marriage Allowance from a lower-earning spouse or civil partner, so your allowance is boosted by £1,260, and the tool nudges your tax-free figure up accordingly. An N means you are the one who gave that £1,260 away, so your allowance shrinks by the same amount. The letters do not change the ten-times arithmetic; they explain why the digits are higher or lower than 1257.
A handful of code prefixes have nothing to do with allowances at all. BR taxes every pound at the basic rate of 20 percent with no allowance, which is the standard code for a second job. D0 taxes the lot at 40 percent and D1 at 45 percent, both used when your allowance and lower bands are already soaked up by other income. A K code is the unusual one: it means deductions such as a company car or untaxed state pension exceed your allowance entirely, so the figure is added to your taxable pay rather than subtracted from it. Enter K500 and the tool reports that £5,000 is treated as extra taxable income for the year.
Decoding 1257L step by step
Here is the default code worked through exactly as the calculator handles it. The arithmetic is trivial, but seeing it laid out makes the K-code and Marriage Allowance variants click into place.
| Step | Result |
|---|---|
| Read the digits in the code | 1257 |
| Multiply by ten for the allowance | 1257 × 10 = £12,570 |
| Apply the suffix (L = standard) | No adjustment |
| Tax-free Personal Allowance | £12,570 |
| Compare: a 1383M code (Marriage Allowance) | £12,570 + £1,260 = £13,830 |
The chart shows how the same £12,570 allowance shifts under three common codes: the standard L, the boosted M, and a BR code that strips the allowance to nil.
A practical tip on Scottish and Welsh codes
If your code starts with an S you are a Scottish taxpayer, and a C prefix marks a Welsh one. This decoder reads the numeric part the same way, because the size of the allowance is identical across the United Kingdom. What differs is the set of rates that allowance sits in front of. Scotland runs its own bands, with a 19 percent starter rate, a 21 percent intermediate rate, and a 42 percent higher rate, so an S1257L taxpayer gets the same £12,570 free but pays a different amount on the income above it. If the tool returns "unrecognised", strip the letter prefix and enter just the digits and suffix.
Why did my code suddenly become K something?
A K code appears when HMRC believes you owe tax on more than your allowance can cover, usually a company car, medical insurance, or an underpayment from a previous year being collected through this year's payroll. There is a legal cap: payroll cannot deduct more than half your pay through a K code in any pay period, so very large adjustments get spread out rather than swallowing a whole payslip.
Is 0T the same as BR?
No. BR taxes everything at 20 percent flat. A 0T code gives no allowance but still applies the proper progressive bands, so higher earnings tip into 40 and 45 percent. HMRC often uses 0T when it has no up-to-date information about you, for example on a final payment after you leave a job, and it can leave you overpaying until the record catches up.