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 one possible reason for a K tax code and should be checked against the information HMRC holds.
- 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 tax code.
- Outdated benefit values or incorrect tax adjustments can result in an incorrect K tax code, so the underlying coding notice should always be checked.
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.
It is important to understand that K154 does not mean £1,540 of additional tax. It means £1,540 is added to the employee’s taxable pay when PAYE is calculated. The actual additional Income Tax depends on the applicable tax rates and the employee’s circumstances.
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.
For a broader explanation of how standard tax codes work, see our 1257L tax code explained guide.
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 other taxable income and adjustments exceed an employee’s Personal Allowance. These are some of the situations that can trigger one.
Benefits in kind (P11D)
A company car, private medical insurance or a beneficial loan can count as taxable benefits. Where benefits are reported through a P11D, the relevant reporting deadline is generally 6 July following the end of the tax year, subject to the applicable reporting rules.
The value of taxable benefits may affect the tax code HMRC issues. Employers should check that the benefits and values recorded are accurate, particularly when a company car changes or a benefit ends during the tax year.
An outdated car value or a benefit that ended partway through the year without the relevant records being updated may contribute to an incorrect tax code. Since the code is calculated using the information HMRC holds, incorrect information can lead to an incorrect result.
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.
HMRC may collect eligible unpaid Income Tax through PAYE, depending on the circumstances and the applicable rules. Not every tax debt or penalty is automatically collected through a K tax code. Employees should check their tax calculation or coding notice to understand the adjustment.
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 may be collected through an employment or private pension code instead.
For a business still employing someone past State Pension age, State Pension income alongside employment income can affect the employee’s available Personal Allowance and tax code. It isn’t necessarily a payroll error when HMRC issues a K code in these circumstances.
Older tax debts
HMRC may include eligible earlier tax underpayments or other adjustments in a tax code where the relevant rules allow it. However, an outstanding late-filing penalty should not automatically be assumed to be the reason for a K code. Check the coding notice or contact HMRC to establish the actual reason.
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 similar; 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.
If the additional £1,540 is taxed at 20%, it represents approximately £308 in additional annual Income Tax before considering the employee’s wider circumstances. This is a simplified illustration, not a guaranteed deduction of £26 every month. Actual tax depends on the employee’s tax position, the applicable rates and how PAYE is calculated during the year.
The 50% cap
HMRC’s rules state that no more than 50% of gross pay can be taken in Income Tax in a single pay period under a K code, whatever the calculation would otherwise produce. Any shortfall may carry forward to a later pay period, and HMRC reconciles the position according to the applicable rules.
The 50% limit is a restriction on the Income Tax deduction made under the K code in a pay period. It does not mean that the employee’s total deductions, including National Insurance and pension contributions, can never exceed 50% of gross pay.
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.
A Week 1 or Month 1 basis generally calculates PAYE using the current pay period rather than the employee’s cumulative pay and tax position. Employers should follow the basis stated in HMRC’s coding notice.
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.
The £50,270 threshold applies to the standard higher-rate threshold in England, Wales and Northern Ireland under the relevant tax rules. Scotland has different Income Tax bands for employment and other non-savings, non-dividend income. The actual effect of a K code depends on the employee’s tax residence and total taxable income.
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.
Use the following checklist when reviewing a K tax code:
- Check the latest HMRC tax code notice and confirm the code and its effective date.
- Review the stated reasons for the adjustment, such as taxable benefits, State Pension income or a previous tax underpayment.
- Compare any benefits in kind with the relevant employee records and P11D information, where applicable.
- Recheck the K code calculation against HMRC’s official guidance if the figures do not appear correct.
- Confirm whether the code is cumulative or operates on a Week 1/Month 1 basis.
- Check that the payroll system applies the 50% limit to the relevant Income Tax deduction.
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.
Employees can also check their tax code through their HMRC Personal Tax Account or the HMRC app. For further guidance, see the official GOV.UK tax codes guide.
What This Means for Payroll and Directors
A wrong K code for a director may result from incorrect benefit information or another adjustment in HMRC’s records, rather than an error in the payroll calculation itself. It starts with what’s been reported to HMRC before the code is even generated.
P11D accuracy is an important check. A changed company car or a benefit that’s ended partway through the year needs to be reflected in the relevant records, since HMRC’s calculation is only as accurate as the information it holds.
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 many potential errors before they compound across several pay periods.
Directors and employers should also review whether the benefit is still provided and whether the relevant taxable value has been reported correctly. If the coding notice appears incorrect, the underlying information should be checked with HMRC rather than changing the code within payroll without authorisation.
For businesses that need help reviewing payroll records, tax codes and employee deductions, Daniel Wolfson & Co provides payroll services.
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 eligible tax underpayment, 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 outdated benefit information or other incorrect adjustments can result in someone paying the wrong amount of tax.
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 timing depends on when HMRC issues the revised tax code and when payroll applies it. Any overpaid tax may be corrected through a subsequent payslip, depending on the tax code basis and the employee’s circumstances. It is not guaranteed that every correction will result in a refund on the very next payslip.
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 can lower future taxable benefit values and may reduce or remove a K code, depending on the other adjustments HMRC includes. However, removing a benefit does not automatically change an existing tax code; HMRC may need updated information before issuing a revised code.
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 benefit details and request a review where appropriate. The employer should continue to follow the latest valid coding notice until HMRC issues a replacement, rather than independently changing the employee’s tax code.
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 one combination that may contribute to a K tax code.
Is a K tax code bad?
No. A K tax code does not automatically mean that HMRC has made a mistake or that the employee has been penalised. It means that the tax adjustments HMRC needs to make exceed the available Personal Allowance. However, employees should check the coding notice to make sure the underlying information is accurate.
What does a K666 tax code mean?
A K666 tax code generally means that £6,660 is added to taxable pay when PAYE is calculated. It does not mean that £6,660 is deducted directly from the employee’s salary or that this amount is the tax owed. The actual Income Tax depends on the employee’s taxable income and applicable tax rates.
How can I check whether my K tax code is correct?
Check your latest HMRC tax code notice, review the benefits and other adjustments included, and compare the figures with your current circumstances. You can also check your tax code through your HMRC Personal Tax Account or the HMRC app. If the information is wrong, contact HMRC or ask your accountant to help review it.
Can a K tax code reduce my take-home pay by more than 50%?
The K code rules limit the relevant Income Tax deduction to 50% of gross pay for a pay period. However, other deductions, such as National Insurance and pension contributions, are separate and may also affect take-home pay.
Does a K tax code affect State Pension payments directly?
The State Pension is generally paid without Income Tax deducted at source. Where tax is due, HMRC may collect it through an employment or pension tax code, depending on the individual’s circumstances.
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