What a K tax code means, why HMRC adds to your taxable pay instead of giving an allowance, and how to check yours, with a 2026/27 worked example.
If your payslip shows a tax code that starts with the letter K, it works backwards from every other code you’ve had. Instead of giving you a slice of tax-free pay, it adds an amount on top of your wages and taxes the lot. That feels alarming the first time you see it, and the deduction is usually bigger than you expect.
Here is the K tax code explained in plain terms: what the letter means, why HMRC gives you one, a real 2026/27 worked example, and how to check whether yours is right.
A K tax code means the things HMRC needs to tax outside your wages, taxable benefits, untaxed income, or tax owed from a previous year, are worth more than your £12,570 personal allowance, so instead of protecting some of your pay, HMRC adds the difference to your taxable income.
Every normal code, like 1257L, multiplies its number by 10 to give you tax-free pay. A K code does the opposite. The number after the K, times 10, is roughly the extra taxable income added across the year. K343 adds about £3,430. K3000 adds about £30,000.
With a standard code your employer protects a slice of your pay each period, then taxes the rest. With a K tax code there is nothing to protect, the allowance has already been used up, so payroll takes the K number, multiplies by 10, splits that across your pay periods, and adds it to your taxable pay before working out the tax.
A worked example makes this concrete.
Tom earns £45,000 a year and has just been given a company car. The taxable value of that car, the benefit in kind, is £16,000 for 2026/27. His benefits (£16,000) are larger than his personal allowance (£12,570) by £3,430. HMRC turns that £3,430 into a negative allowance, drops the last digit, and issues code K343.
Here is what K343 does, month by month, paid monthly:
Compare that to the same £45,000 on tax code 1257L, where £1,047.50 is protected each month and the tax is £540.50. The K code costs Tom about £267 more a month, which is £3,200 over the year, exactly 20% of his £16,000 company car benefit. The code is not punishing him, it is collecting the tax on the car that no payslip would otherwise touch.
One thing the K code does not do is change his National Insurance. NI is worked out on what Tom actually earns, so it stays the same whether his code is 1257L or K343.
There are three common reasons HMRC issues a K tax code, and the first is by far the most frequent.
Taxable benefits worth more than your allowance. A company car is the classic trigger, especially a higher-emission petrol or diesel model where the benefit in kind can run to £15,000 or £20,000 a year. Add fuel provided for private use, private medical insurance, or an interest-free loan, and the total can sail past £12,570. Once it does, your allowance flips negative and you get a K code.
Untaxed income that uses up the allowance. This catches a lot of people drawing the State Pension while still working or holding a private pension. The State Pension is taxable but paid without tax taken off, so HMRC collects the tax through your other income. If the State Pension alone is close to or above £12,570, your allowance is spoken for, and a K code mops up the rest.
Tax owed from a previous year. If HMRC finds you underpaid in an earlier year, through a P800 calculation, Self Assessment, or cross-checked bank interest, they often claw it back by cutting your allowance. A small underpayment usually just lowers your code below 1257L. A larger one can push it into K territory.
A K tax code is correct when the benefit or debt behind it is real and current. If you genuinely drive a company car worth £16,000 a year, K343 is doing exactly what it should, and switching you back to 1257L would just leave you with a tax bill at the end of the year.
It is wrong, and worth fixing fast, when the trigger has gone but the code has not caught up. Handed the company car back but still on a K code? You are overpaying every month. Paid off last year’s underpayment but the code never reverted? Same problem. Benefits also change in value each April, so a code set for an old car or an old medical premium can quietly over-deduct.
It helps to put the two side by side, because they sit at opposite ends of the same scale.
Tax code 1257L gives you £12,570 of tax-free pay across the year. It is the default for most UK workers with one job and no complications. A K3000 tax code does the reverse: it adds about £30,000 of taxable income on top of your actual wages before any tax is worked out. To land on K3000 you would need benefits, untaxed income, or tax owed of roughly £42,570, your £12,570 allowance plus the £30,000 that gets added. That is rare, and usually means a very valuable company car with private fuel, or a sizeable untaxed pension, or a big underpayment being recovered.
So if 1257L feels like HMRC handing you a tax-free cushion, a K tax code feels like the cushion being removed and an extra bill stacked on top. Same machinery, opposite direction.
There is one safeguard worth knowing. The tax collected through a K tax code can never be more than half of your gross pay for that period. This is the overriding limit, and it stops a very high K code from swallowing an entire pay packet. If your code would take more than 50%, HMRC carries the rest over to the next period instead.
To check whether your K tax code is right, go to the source rather than guessing:
If the benefit listed is wrong, out of date, or gone, you can update it in the app or call the Income Tax helpline on 0300 200 3300. HMRC will reissue the code, and any overpayment usually comes back through a later payslip or after the tax year ends.
The maths behind K codes has not changed for 2026/27. The personal allowance is still £12,570, frozen until April 2028, and the basic-rate band still runs to £50,270. So the way a K code is calculated, deductions minus allowance, drop the last digit, is the same as last year.
What did change, from April 2026, is how you hear about it. HMRC moved most coding notices to digital-first, so a new K code often lands in your online account rather than in the post. That is why some people saw their code switch to a K code in spring without warning, the push notification or email was easy to miss. If your April or May 2026 payslip suddenly took a lot more tax, a new K code in your HMRC account is the first thing to check.
If you want to see what your take-home should actually be on a K tax code, with the benefit in kind, National Insurance, pension, and student loan all worked out, the NetPay app does the sums for you. Free to download.
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A K tax code means your untaxed income or benefits are worth more than your tax-free personal allowance, so HMRC adds the difference to your taxable pay instead of subtracting an allowance. The number after the K, multiplied by 10, is roughly how much extra taxable income is added across the year. It usually appears when you have a company car, other taxable benefits, untaxed income, or tax owed from a previous year.
Not necessarily. A K tax code is not a penalty, it is just HMRC collecting tax on income or benefits that are not taxed at source. If you genuinely have a company car or owe tax from last year, a K code is correct and is doing its job. It is only a problem if the benefit or debt behind it is wrong or out of date, which is worth checking on the HMRC app.
HMRC adds up your taxable benefits and any untaxed income or tax owed, then subtracts your personal allowance. If the deductions are larger than the allowance, the leftover becomes a negative allowance. Drop the last digit and put a K in front to get the code. So a £3,430 negative allowance becomes K343, which adds about £3,430 to your taxable pay over the year.
No. A K tax code only changes how much income tax you pay. National Insurance is worked out on what you actually earn each pay period, not on your tax code, so it is unaffected. If your take-home dropped after a code change to a K code, the extra deduction is income tax, not NI.
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A note on financial advice: NetPay UK calculates take-home pay based on official HMRC tax rules. This article reflects rules in force at the time of publication (6 June 2026). Tax rules change. For complex situations, consult a qualified UK accountant or visit gov.uk/income-tax.
