What Does a K Tax Code Mean?

A K tax code is unusual because it works in reverse. Instead of giving you a tax-free amount, it adds to your taxable income. You get a K code when the untaxed income or benefits you receive are worth more than your Personal Allowance, so HMRC needs to collect extra tax through your pay.

In this insight we explain what a K code means, why you might have one, how the number works, and the 50% rule that stops too much tax being taken from any single payday.

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Key Takeaways

• A K tax code adds an amount to your taxable income rather than giving you a tax-free allowance, because your untaxed income or benefits exceed your allowance.
• The number after the K, multiplied by 10, is roughly the extra amount added to your taxable pay across the year.
• Common causes include valuable benefits in kind, such as a company car, tax owed from a previous year, or a State Pension that uses up your allowance.
• A 50% overriding limit means no more than half of your pay from that source can be taken in tax in any single pay period.

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How a K Code Is Different

Most tax codes give you a tax-free amount, but a K code does the opposite, adding an amount to your taxable pay because your deductions are greater than your allowance. That is why the letter comes at the front of the number, such as K475, rather than after it.

This happens when the value of untaxed income or benefits is more than the Personal Allowance you would normally receive. Rather than a negative allowance being ignored, HMRC turns it into extra taxable income so the right amount of tax is collected through PAYE. For the wider context on how codes work, see our guide to tax codes explained.

Why You Might Have a K Code

Several situations can lead to a K code:

• Valuable benefits in kind. A company car, fuel, or private medical insurance that is worth more than your allowance.
• Tax owed from a previous year. HMRC may collect an underpayment through your code, which can tip it into K territory.
• State Pension using your allowance. If your State Pension is more than your Personal Allowance, the excess can be collected against other income through a K code.
• Multiple sources of income where deductions build up beyond the allowance.

If your code has just switched to K, our guide to why your tax code has changed explains how these adjustments come about.

How the Number Works

With a standard code, you multiply the number by 10 to get your tax-free amount. With a K code, you multiply the number by 10 to get the extra amount added to your taxable income for the year.

For example, a K475 code adds roughly £4,750 to your taxable pay across the year. So instead of having tax-free pay, you are treated as having additional income to be taxed. Your employer spreads this across your paydays, just as they would spread an allowance, but in the opposite direction.

To see how this affects your take-home, our income tax calculator can help you model the impact of extra taxable income.

The 50% Overriding Limit

Because a K code adds to your taxable pay, there is a built-in protection so it cannot swallow your whole wage. The 50% overriding limit means your employer cannot deduct more than half of your gross pay from that source in tax in any single pay period.

If the K code would otherwise take more than 50%, the excess is carried forward and collected later, rather than leaving you with almost nothing to live on that payday. This spreads the tax more manageably while still ensuring it is paid over time.

Checking a K Code Is Correct

K codes deserve a careful look, because they mean you are paying tax on more than your salary alone. To check yours:

• Read your coding notice (form P2), which lists the deductions that pushed your code into K.
• Confirm the benefits shown match what you actually receive, and that their values look right.
• Check any underpayment being collected is genuine and correctly calculated.
• Use your personal tax account on GOV.UK to review the detail and contact HMRC if anything is off.

If a benefit has ended or a figure is wrong, correcting it can move you off the K code. You can find more tools to help on our calculators page, and if you also want to understand cash you receive from family, our guide on whether you need to declare cash gifts to HMRC may be useful.

Frequently asked questions

Is a K tax code bad?

It is not a penalty, but it does mean you are being taxed on more than your salary, because your untaxed income or benefits exceed your allowance. It is worth checking the figures behind it are correct, as an out-of-date benefit or wrong underpayment could be pushing your tax higher than it should be.

How much extra tax does a K code take?

It depends on the number. Multiply the K number by 10 to get roughly the extra taxable amount for the year, then apply your tax rate. For example, K475 adds about £4,750 of taxable income, so a basic-rate taxpayer would pay around 20% of that over the year, spread across paydays.

What is the 50% limit on a K code?

It is a safeguard that stops more than half of your pay from that source being taken in tax in any one pay period. If the K code would take more, the extra is carried forward to later paydays, so you are never left with less than half your gross pay from that income.

Can a K code change back to a normal code?

Yes. If the reason for it ends, such as giving up a company car or clearing an underpayment, HMRC can issue a normal code with a tax-free allowance again. Keeping your details up to date through your personal tax account helps make sure the change happens promptly.

General information only, not financial advice. If you think your tax code is wrong, contact HMRC.