A K tax code adds untaxed income to salary or pension before HMRC calculates tax, rather than subtracting a Personal Allowance from it. HMRC issues this code when deductions or untaxed income exceed the standard tax-free allowance for that year. The letter K signals a negative allowance: instead of reducing taxable income, it increases the amount treated as taxable.
This guide explains why HMRC applies this code, how the calculation works, and what to check if one appears on a payslip.
Why Does HMRC Issue a K Tax Code?
HMRC applies this code when untaxed income or deductions exceed a person’s Personal Allowance for the year. Common triggers include:
- Company benefits: a company car, private medical insurance, or other benefits in kind that HMRC has not yet taxed elsewhere
- State Pension income: paid without tax deducted at source, so HMRC collects the tax through the employment code instead
- Unpaid tax from a previous year: HMRC recovers an underpayment by adjusting the current code
- Large untaxed savings interest: in less common cases, where other collection methods do not apply
HMRC calculates the excess, then issues a code that reflects the shortfall rather than an allowance.
How Does a K Tax Code Affect My Pay?
A K tax code increases the amount of income treated as taxable, rather than reducing it. For example, a code of K475 adds £4,750 to taxable pay before tax is calculated on the total. This differs sharply from a standard code such as 1257L, which removes £12,570 from taxable income before tax applies, and from a BR tax code, which gives no allowance at all but adds nothing either.
Work expense relief moves a code in the opposite direction. A tool tax rebate, for example, can raise the tax-free amount in a code rather than reduce it.
Tax deducted under a K code cannot exceed 50% of pay in any single pay period, even if the calculation would otherwise require it. HMRC applies this limit to protect take-home pay, and any tax left uncollected is recovered later in the year or after the year ends.
Is a K Tax Code Always Correct?
Not necessarily. A K code reflects HMRC’s current understanding of benefits, pension income, or unpaid tax, and this information is not always accurate. Common causes of an incorrect K code include:
- Outdated benefit-in-kind figures, such as a company car changed or returned without HMRC’s record updating
- A State Pension estimate that does not match the actual amount received
- An underpayment that has already been settled through another route
Checking the breakdown behind the code, available through the Personal Tax Account, confirms whether the figures match actual circumstances.
How Do I Check or Query My K Tax Code?
HMRC sends a coding notice explaining how it calculated a K code, showing each item added to taxable income. The notice arrives by post or appears in the Personal Tax Account online.
Anyone who disagrees with the figures should contact HMRC directly, with details of the benefit, pension, or underpayment in question. HMRC then reviews the calculation and issues a corrected code if the figures need adjusting.
For employers, keeping benefit-in-kind figures reported to HMRC current each year reduces the chance of an incorrect K code reaching an employee’s payslip. Businesses using our payroll services in Slough have these figures checked as part of each year’s P11D reporting.
Can a K Tax Code Result in a Refund?
Yes, in some circumstances. If HMRC later reduces the untaxed income figure behind the code, or if the code was wrong from the start, too much tax may have been collected. HMRC then issues a refund once the correction goes through, either automatically or after a query is raised.
A K code that turns out to be accurate, reflecting genuine benefits or unpaid tax, does not generate a refund, since the tax collected matches the actual liability.
What Happens If My K Tax Code Changes During the Year?
HMRC updates a K code whenever the underlying figures change, for example when a company benefit ends or a new one starts. The updated code applies from the next available pay run, and payroll recalculates deductions going forward rather than adjusting past pay periods retroactively, unless HMRC specifically instructs otherwise.
Frequently Asked Questions
What does the K stand for in a K tax code? The K does not stand for a specific word; it signals that deductions exceed the Personal Allowance, so tax applies to a higher figure than gross pay.
Can self-employed people have a K tax code? Not on their self-employment income. K codes apply only through PAYE, to employment or pension income. Self-employed profits are taxed separately through a Self Assessment tax return, although a self-employed person who also has a job or a pension can receive a K code on that income.
Does a K tax code mean I owe HMRC money? Not always. It often reflects benefits or pension income that reduce the available allowance, rather than an unpaid tax debt specifically.
Is there a maximum K tax code? There is no fixed maximum figure, but the 50% deduction limit caps how much tax is taken from any single pay period, regardless of how high the code number is.