A K tax code is issued by HMRC when an employee’s taxable benefits or other tax adjustments exceed their available Personal Allowance. Instead of increasing the tax-free amount, it adds an amount to taxable pay so the correct Income Tax can be collected through PAYE.
For employers, payroll teams and accountants, a K tax code affects more than payroll calculations. It can result from benefits in kind, tax underpayments or other HMRC adjustments, making it important to verify that the information behind the code is accurate before applying it.
This guide explains what a K tax code means, why HMRC issues one, how it is calculated, how it affects payroll and the checks employers should carry out before processing a coding notice.
Key takeaways
- A K code follows a fixed formula: (Allowance − Benefit) → drop the last digit and sign → reduce by 1 → add K.
- Benefits in kind on a P11D, particularly a company car, are the most common driver for directors and the most common source of an incorrect code.
- No more than 50% of gross pay can be deducted in a single pay period under a K code.
- Class 1A National Insurance on benefits is calculated separately and isn’t affected by a K code.
- An outdated P11D value is the most common reason a K code turns out wrong.
What a K Tax Code means and how HMRC calculates it?
A K code means the deductions HMRC needs to make exceed the Personal Allowance, so HMRC adds the excess to taxable pay instead of reducing it.
HMRC’s calculation: Allowance − Benefit value, drop the last digit and the minus sign, reduce by one, add K.
Example: £12,570 allowance − £14,120 company car benefit = −£1,550 → 155 → 154 → K154.
A few things worth knowing about this calculation, not always obvious from the code itself:
- K154 adds £1,540 to taxable pay before tax is calculated (154 × 10), the reverse of a standard code like 1257L, which gives £12,570 tax-free.
- The “reduce by one” step is easy to miss when rechecking a code manually and it’s the most common reason a manual recalculation ends up a single digit out.
- Class 1A National Insurance on the same benefits is calculated separately and isn’t affected by the code.
Once the calculation is clear, it’s easier to see how a K tax code differs from the other PAYE tax codes employers deal with regularly.
How does a K Tax Code compare with other common PAYE Tax Codes?
A K tax code differs from other PAYE tax codes because it increases taxable pay instead of reducing it. While codes such as 1257L, BR, D0 and D1 apply a Personal Allowance or a specific Income Tax rate, a K tax code adds taxable benefits or other HMRC adjustments to PAYE income. For employers and payroll teams, this means a K tax code often requires additional checks before it is applied.
The table below highlights how a K tax code compares with some of the most commonly used PAYE tax codes.
| Tax Code | Meaning | Commonly used when |
|---|---|---|
| 1257L | Standard Personal Allowance applies. | Most employees with one job and no tax adjustments. |
| BR | All PAYE income is taxed at the basic rate. | Often used for a second job or pension. |
| D0 | All PAYE income is taxed at the higher rate. | Used when all income from a job is taxable at the higher rate. |
| D1 | All PAYE income is taxed at the additional rate. | Used when all income from a job is taxable at the additional rate. |
| K | Taxable benefits or other adjustments exceed the employee’s Personal Allowance, increasing taxable pay. | Common where employees receive significant benefits in kind or HMRC collects additional tax through PAYE. |
Recognising how a K code differs from other PAYE tax codes is only part of the process. Employers and accountants also need to understand what has caused HMRC to issue it, since that determines whether the code reflects the employee’s current circumstances.
Why does HMRC use a K Tax Code?
HMRC issues a K code when taxable benefits, prior-year tax owed, State Pension income or older tax debts exceed an employee’s Personal Allowance. These are the four situations that commonly trigger one.
Benefits in kind (P11D)
A company car, private medical insurance or a beneficial loan all count as taxable benefits, valued and reported on a P11D by 6 July following the tax year they cover. Once their combined value exceeds the Personal Allowance, HMRC issues a K code for the difference.
The most common reason a K code turns out wrong sits here: an old car value still on file after a change or a benefit that ended mid-year without the P11D being updated to reflect it. Since the code is calculated from whatever value HMRC holds, an outdated P11D produces a technically correct calculation from an incorrect starting number.
Tax owed from a previous year
If HMRC calculates an underpayment, usually shown on a P800, it can collect the shortfall through the tax code rather than requesting a lump sum. This is worth distinguishing from a benefits-driven K code, since correcting it means resolving the underlying P800, not updating a P11D.
State Pension income
The full new State Pension for 2026/27 is £241.30 a week, £12,547.60 a year, just £22.40 below the £12,570 Personal Allowance. HMRC can’t deduct tax from the State Pension directly, since it’s paid gross by the DWP, so any tax due is collected through an employment or private pension code instead.
For a business still employing someone past State Pension age, a modest income on top of the pension is often enough on its own to trigger a K code and it isn’t a payroll error when it does.
Older tax debts
HMRC can also use a K code to collect older debts, such as an unresolved late filing penalty, through PAYE rather than direct payment and must notify the employee before doing so.
Ordinarily, HMRC doesn’t include non-PAYE income, such as self-employment income already returned on a Self Assessment return, within a K code calculation; where it appears to have been included, that’s usually worth querying directly with HMRC rather than assumed correct.
Employees in Scotland or Wales may see an SK or CK prefix instead. The mechanics are identical; only the income tax rates applied differ.
For payroll teams, the reason behind the code matters because it determines what should be checked before the next payroll run.
How a K Tax Code affects Payroll?
Using the K154 example on a £30,000 salary, taxable pay rises to £31,540 once the code is applied, adding roughly £308 to the year’s tax bill, around £26 a month, spread evenly rather than taken as one deduction.
The 50% cap
No more than 50% of gross pay can be taken in a single pay period under a K code, whatever the calculation would otherwise produce. Any shortfall carries forward to the next pay period and HMRC reconciles the balance at year end.
Non-cumulative codes (W1 or M1)
A W1 or M1 suffix changes the timing rather than the amount, applying the code on a non-cumulative basis rather than the employee’s year-to-date position. HMRC uses this when a new or amended code arrives mid-year, to avoid a large catch-up deduction landing in one go.
Moving into a higher tax band
A sizeable K code can move part of income from the 20% to 40% band if salary is already close to the £50,270 higher-rate threshold.
How to check a K Tax Code?
Before applying a new K tax code, employers should confirm that the information used by HMRC is accurate. This helps prevent incorrect tax deductions and reduces the need for later payroll adjustments.
The verification process below provides a quick reference for the key checks to complete before processing a new coding notice.
If any of these checks reveal that the coding notice is based on outdated or incorrect information, the next step is to have the tax code reviewed.
If the information does not reflect the employee’s current circumstances, the employer should notify HMRC where appropriate or ask the employee to contact HMRC to request a revised tax code.
Payroll must apply the latest coding notice issued by HMRC and cannot amend a tax code independently. Putting these checks into the payroll process helps employers avoid unnecessary corrections and gives directors confidence that the right amount of tax is being collected.
What this means for Payroll and Directors?
A wrong K code for a director is usually a P11D problem, not a payroll problem. It starts with what’s been reported to HMRC before the code is even generated.
P11D accuracy is the first check. A changed company car or a benefit that’s ended partway through the year needs updating on the P11D as it happens, since HMRC’s calculation is only as accurate as the value it’s given. This is the single most common reason a director ends up on the wrong K code.
The second is treating an unexplained K code change as something to verify, not absorb. If a code appears or changes without an obvious cause on payroll’s side, confirming with the employee whether they’ve received a coding notice and checking the figure against the calculation above, catches most errors before they compound across several pay periods.
Conclusion
A K tax code isn’t a sign something’s gone wrong. It’s a calculated mechanism for collecting tax that a normal allowance can’t cover, whether that’s a benefit in kind, a State Pension or an old debt and the calculation behind it follows a fixed, checkable formula.
What matters for payroll and the accountant reviewing it is whether the figure HMRC used to calculate the code is still accurate, since an outdated P11D value is the usual reason someone ends up paying more than they should.
Daniel Wolfson & Co’s personal tax and payroll team can review a coding notice against the underlying P11D or P800 and deal with HMRC directly on a client’s behalf. Book a consultation now or email us at office@danielwolfson.co.uk to arrange a review.
FAQs
Can an accountant get an incorrect K code corrected without the director doing anything?
Yes, with authority to act on the client’s behalf. An accountant can query the figure with HMRC directly, though HMRC may still need confirmation from the director on details like a benefit that’s ended.
How quickly does correcting a K code change take-home pay?
The correction applies from the next payroll run after HMRC issues the updated code. Any overpaid tax is refunded through that same payslip rather than held until year end.
Does a K code apply to dividends as well as salary?
No, a K code only affects PAYE income, so it changes tax on a director’s salary but has no effect on how dividends are taxed.
Can a K code be avoided by reducing a director’s benefits in kind?
Reducing or removing a benefit lowers future P11D values and can reduce or remove a K code from the next tax year, though it doesn’t change a code already calculated for the current year.
What should a business do if HMRC issues a K code the business believes is based on outdated figures?
Contact HMRC directly with the current P11D or benefit details and request a recalculation. Paying under an incorrect code doesn’t fix itself and the longer it runs, the more pay periods need correcting retrospectively.
Is a K tax code more common for directors than other employees?
It’s not exclusive to directors, but directors are more likely to have benefits in kind, such as a company car, sitting on top of a lower PAYE salary, which is the combination most likely to exceed the Personal Allowance.
About the Author
