Thirtyish

Your tax code is an instruction to your employer

You're thirtyish and there's a short string of numbers and letters on your payslip that nobody has ever explained to you.

It's not a rating, a status or a judgement about you. It's an instruction to your employer, telling them how much of your pay to leave untaxed before they start deducting.

Most people are on 1257L, which means the standard tax-free allowance. If yours is something else, it changes your income tax — and only your income tax. That last part matters more than it sounds, and I'll come back to it.

The number is your tax-free pay with the last digit dropped

1257L means £12,570 of your pay each year isn't taxed.

Add a zero to the number and you've got the amount your employer leaves alone across the year, spread evenly across your paydays. A smaller number means less tax-free pay and more tax. A bigger number means the opposite.

Numbers move for ordinary reasons: a benefit like private medical cover, tax owed from an earlier year being collected a bit at a time, or an allowance moved to or from a partner. None of those means anything has gone wrong.

The letter says why, not how much

L, M, N and T are all ordinary allowance codes. They differ in the reason, not the effect.

L is the standard one. M and N mean you've transferred part of your allowance to a partner, or received part of theirs. T means HMRC has something in your code they review. All four work the same way: the number is your tax-free pay, and the letter is just filing.

There's one more in the same family. 0T means no tax-free pay at all, so every pound is taxed. It usually turns up when a new employer hasn't got your details yet, and it usually sorts itself out.

BR, D0 and D1 tax everything, and usually mean a second job

They apply one flat rate to all of your pay, with no tax-free allowance.

BR taxes everything at the basic rate. D0 taxes everything at the higher rate. D1 taxes everything at the additional rate.

They exist because your tax-free allowance can only be given once. If you've got two jobs, one of them gets your allowance and the other is taxed from the first pound. That's all these codes are doing.

Which is why this calculator is careful with them. If you're on BR, D0 or D1 here, I can tell you exactly what comes off this job's pay. I can't tell you your overall position, because I never asked about your other income. The figure will be right, and it'll be answering less than your whole question — and it says so on screen rather than leaving you to spot it.

A K code is the same idea running backwards

Instead of tax-free pay, it adds pay you never received.

Sometimes the things HMRC needs to tax add up to more than your allowance covers: a large company benefit, a company car, tax owed from an earlier year, or a state pension being taxed through a job. When there's more to tax than there's allowance to soak it up, the code flips.

A K code adds an amount to your taxable pay. K475 means an extra £4,750 across the year is treated as taxable, on top of what you were actually paid. You never received it. It's a device for collecting tax on something else.

There's a legal limit on how far this can go. Your employer can't take more than half your gross pay in income tax in any pay period, whatever your code says. That cap is the law, and it applies to every code — K codes are simply the only place it ever comes up. If it bites, the rest isn't written off; it's collected later. The calculator applies the cap and tells you when it has.

K codes don't touch your National Insurance or your student loan

The added pay isn't earnings. Nobody paid it to you.

This is where a lot of guesswork goes wrong, so it's worth being exact. National Insurance is charged on your earnings. Your student loan is worked out on the same earnings — HMRC defines it that way. A K code doesn't add to your earnings. It adds to your taxable pay, which is a different thing.

So a K code raises your income tax and leaves your National Insurance and your student loan repayment exactly where they were. Three deductions, running off two different figures.

Scotland's codes look like the rest of the UK's and don't mean the same things

SD0 is not D0. This is the trap.

A code starting with S means you pay Scottish income tax. Scotland has more bands than the rest of the UK, so it has more flat-rate codes — and the letters don't line up.

Assume SD0 is Scotland's version of D0 and you'll land on the wrong rate — and it's a plausible enough mistake that the answer still looks perfectly reasonable. There's no code for Scotland's starter rate.

Welsh codes start with C, and get the same answer

C means Wales sets your income tax. Wales currently sets it to match the rest of the UK.

Since 2019 the Senedd has set part of the income tax rate in Wales, and every year so far it's chosen rates identical to England and Northern Ireland's. So a C code gets the same answer as an unprefixed one, and the calculator accepts C codes rather than rejecting them.

That's a choice, not a law. If Wales ever sets different rates, this stops being true.

W1, M1 or X on the end changes less than you'd expect

On a steady salary, nothing. On a bonus month, quite a lot.

If your code ends in W1, M1 or X, it's non-cumulative. Your tax is worked out on that payslip alone, as GOV.UK puts it, "as if you're paid that amount every week or month of the year", rather than being balanced across the year so far.

Here's the surprise: if your pay is the same every month, that gives exactly the same figure as a normal code. Taxing one steady month as though it repeated all year is the same sum as taxing the year and dividing by twelve. There's nothing to correct, so nothing looks different, and the figure here is right for you as it stands.

It stops being true the moment your pay isn't steady. A bonus month on a non-cumulative code is taxed as though that inflated month happened every month of the year, and unlike a normal code nothing corrects it during the year. It comes back after the tax year ends, usually automatically when HMRC reconciles the year, though you can chase it if it doesn't. If you're getting a bonus and your code ends in W1, M1 or X, the calculator's bonus section is worth reading.

Where this breaks down

What this page and the calculator don't handle.

I can't tell you whether your code is right. I take it as given and apply it. If it's wrong you're paying the wrong tax, and your personal tax account is where to check it.

If you earn over £100,000 your allowance tapers away, and HMRC may already have built that into the number in your code. I apply the taper on top of what you give me, so if your code already reflects it, the figure here is harsher than reality. The calculator flags that rather than hiding it.

NT means no tax at all. It's rare and it has specific causes: the calculator accepts it and deducts no income tax, but if you're on one you've probably got a situation this tool is too simple for.

And the whole page assumes one job. A second income is out of scope here, which is exactly why the BR, D0 and D1 warning above exists.

I can't tell you what to do about your own pay. I don't know your situation, and this is information rather than advice.

2026/27 tax year, from GOV.UK. What this calculator assumes and doesn't handle.