A K tax code means your income adjustments exceed your personal allowance. HMRC collects the extra tax by reducing your tax-free pay — sometimes to zero.
50% -- that is the maximum amount HMRC can take from any single pay packet when you have a K tax code. A K code is one of the most alarming codes to see on your payslip because it works in reverse: instead of giving you a tax-free allowance, it adds to your taxable income. HMRC uses it when your deductions (company car, benefits in kind, underpaid tax from a previous year, or state pension) exceed your Personal Allowance entirely.
With most tax codes, the number represents your tax-free Personal Allowance. With a K code, the number represents the amount being added to your taxable income. The calculation reverses.
How it works:
Example: You have a company car with a Benefit in Kind value of £8,400 and private medical insurance worth £5,000. Total deductions: £13,400. Your Personal Allowance is £12,570. The excess is £830. HMRC issues code K83. This means £830 is added to your taxable income above your actual salary.
K codes are less common than standard codes, but they are not unusual. Here are the scenarios that produce them:
Company car and benefits in kind. If your employer provides a car, private medical insurance, gym membership, interest-free loans above £10,000, or other taxable benefits, each one reduces your effective allowance. A high-value company car alone can push you into K code territory. A petrol car worth £40,000 with a 37% BIK rate generates a £14,800 benefit, already exceeding the entire Personal Allowance.
State Pension plus employment. If you work while receiving the State Pension, the pension is taxable but not taxed at source. HMRC collects the tax by adjusting your employment tax code. The full new State Pension is £11,502 per year (2025/26). Added to a company car benefit, it can easily exceed the £12,570 allowance.
Underpaid tax being collected. If HMRC discovers you underpaid tax in a previous year (from a wrong code, unreported income, or estimated figures that proved too low), they often recover the debt by adjusting your current code. If the amount being collected, combined with any benefits, exceeds your allowance, you get a K code.
Multiple taxable benefits stacking up. Individually, each benefit might seem small. But company car, fuel benefit, medical insurance, and underpaid tax collection can combine to create a significant K code adjustment.
Here is how a K497 code affects an employee earning £45,000:
Step 1: Calculate the K code addition. K497 means £4,970 is added to taxable income (497 multiplied by 10).
Step 2: Calculate total taxable income. £45,000 (salary) plus £4,970 (K code addition) = £49,970 total taxable income.
Step 3: Apply income tax bands.
| Band | Income in band | Rate | Tax |
|---|---|---|---|
| Basic rate | £49,970 | 20% | £9,994 |
| Total tax | £9,994 |
Step 4: Compare to 1257L. On 1257L, the same £45,000 salary would produce taxable income of £32,430 (after deducting £12,570 allowance) and a tax bill of £6,486.
The K code difference: £9,994 minus £6,486 = £3,508 more tax per year, or about £292 more per month.
Note: the K code does not mean you pay tax twice on the benefit itself. The benefit has already been valued and included in the K code number. The employer simply collects more tax through PAYE to cover the total liability.
HMRC has a built-in safeguard for K codes: your employer cannot deduct more than 50% of your gross pay in any single pay period. This is called the "regulatory limit" or the 50% rule.
What this means in practice:
Why this matters: The 50% rule means you will always take home at least half of your gross pay in any given month, no matter how large the K code adjustment is. This protection exists specifically because K codes can create very high effective tax rates.
Step 1: Review your P2 coding notice. HMRC sends a P2 every time your code changes. It itemises every allowance and deduction. Check that each line is accurate: the company car BIK value, the medical insurance value, any underpaid tax amount.
Step 2: Verify benefit values with your employer. Ask your HR or payroll department for a copy of your P11D (the annual statement of benefits and expenses). Compare the figures on the P11D with those on your P2 notice. If they do not match, one of them is wrong.
Step 3: Check for old deductions. Sometimes HMRC continues to collect underpaid tax or includes benefits you no longer receive. If you returned a company car two years ago but it still appears on your P2, your K code is inflated.
Step 4: Log into your HMRC Personal Tax Account. Go to "Check your Income Tax" and review the current year estimates. HMRC shows each item affecting your code and its value.
If your K code is wrong, you have the same options as any tax code dispute:
Online: Update your HMRC Personal Tax Account at gov.uk/personal-tax-account. Report changes to your benefits, employment, or other income.
By phone: Call HMRC on 0300 200 3300 (Monday to Friday, 8am to 6pm). Explain which deductions are incorrect and why. HMRC can issue a corrected code to your employer within days.
Through your employer: If the error is in the P11D (wrong BIK value), ask your employer to file a corrected P11D with HMRC. This will trigger an automatic code update.
If you believe you have overpaid tax because of an incorrect K code, the overpayment will be refunded cumulatively once the code is corrected. For large overpayments spanning multiple months, the refund may arrive as a single lump sum in one pay packet.
Panicking and assuming it is wrong. A K code can be perfectly correct if you have high-value benefits in kind. Before calling HMRC, check your P2 notice and P11D to see exactly what is driving the code.
Not updating when benefits change. If you return a company car, cancel private medical insurance, or stop receiving a taxable benefit, HMRC will not know unless your employer reports the change. Chase your employer to update the P11D.
Ignoring the 50% rule underpayment. If the 50% cap prevents your employer from collecting all the tax due in a given month, the shortfall accumulates. At year end, you may owe the balance. Budget for it.
Confusing K code with emergency tax. A K code is not an emergency code. It is a calculated, intentional adjustment by HMRC based on your specific deductions. Emergency codes (W1, M1, 0T) are temporary and usually resolve themselves. K codes persist until the underlying deductions change.
Forgetting about state pension. Many people are surprised to get a K code when they start drawing their State Pension while still employed. The pension is taxable income that HMRC collects through your employment code. At £11,502 per year (2025/26 full rate), it can significantly reduce or eliminate your Personal Allowance.
Related tax codes: 1257L tax code | BR tax code | 0T tax code | T tax code
TapTax connects to your bank, categorises expenses automatically, and submits quarterly updates to HMRC. Free plan, no card required.
