Answers · UK 2025/26

How does a K tax code work and why would I have one?

A K tax code is used when your total deductions (such as unpaid tax from a previous year, or the taxable value of benefits) exceed your tax-free allowances, meaning HMRC needs to add extra taxable income rather than subtract an allowance. Instead of reducing your taxable pay, a K code effectively increases it -- but total deductions in any single pay period are capped at 50% of your gross pay.

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Most UK tax codes (like 1257L) represent an allowance to be deducted from your pay before tax is calculated. A K code works in reverse: the number represents an amount to be ADDED to your taxable pay, because your negative adjustments outweigh your normal Personal Allowance. **When a K code arises** HMRC typically issues a K code when: - The taxable value of employment benefits (company car, private medical insurance, other benefits in kind) exceeds your Personal Allowance - You owe tax from a previous year that is being collected through your current tax code - You are receiving the State Pension alongside employment income, and the State Pension (which is taxable but paid without deduction of tax) uses up your entire Personal Allowance and more **Worked example** Priya has a company car with a taxable benefit value of £14,500, but her Personal Allowance is only £12,570. The excess (£14,500 − £12,570 = £1,930) becomes the basis for a K code: K193 (the code number is the excess divided by 10, rounded). Instead of deducting an allowance, HMRC add £1,930 to her salary before working out tax. If Priya's salary is £45,000, her taxable pay for tax purposes becomes £45,000 + £1,930 = £46,930, and Income Tax is calculated on that higher figure. **The 50% deduction cap** To prevent K codes from taking an excessive chunk of any single payslip, HMRC caps total tax deductions under a K code at 50% of your gross pay in that period. If the calculated tax would exceed 50% of gross pay, the excess is carried forward and collected later in the tax year (or via Self Assessment if not fully collected). **Why K codes catch people out** Because a K code effectively means MORE tax is taken than a straightforward 0% or 20% deduction from a positive allowance would suggest, people sometimes assume a payroll error has occurred when in fact the code is correctly reflecting benefits or historic underpayments being clawed back. **What to do if you get a K code** Check your HMRC Personal Tax Account or the accompanying coding notice to see exactly what benefits or adjustments make up the K code. If a company car has been removed, private medical insurance has ended, or a previous employer over-reported a benefit, you should contact HMRC to have the code corrected, since an outdated K code can significantly overtax you.