Estimate tax for a UK K code
Enter your pay details to estimate how a K tax code changes your PAYE income tax. This tool uses UK (England/Wales/Northern Ireland) income tax bands for a quick estimate.
Note: This is an estimate for income tax only (not National Insurance, student loan, or benefits in kind adjustments beyond your K code).
What a K tax code means
A K tax code usually means your deductions (for example, untaxed income from benefits, state pension, or underpaid tax from a previous year) are higher than your allowances. Instead of giving tax-free pay, a K code creates a negative allowance.
In plain English: HMRC treats part of your pay as extra taxable income so more PAYE tax is collected through payroll.
How this K code calculator works
1) Decode the code
The number in the code is multiplied by 10.
- K500 = £5,000 extra taxable income per year
- K125 = £1,250 extra taxable income per year
2) Build estimated taxable income
The calculator annualises your pay, subtracts any pre-tax pension amount you enter, and then adds the K-code adjustment:
Estimated taxable income = Annual pay after pension + (K number × 10)
3) Apply PAYE tax bands
The estimate then applies UK income tax bands (basic, higher, additional) to calculate annual tax and converts that to your selected pay frequency.
4) Show the 50% K-code safeguard
PAYE systems generally limit K-code tax deductions so tax taken for a pay period does not exceed around 50% of that period’s gross pay. This tool shows both the uncapped estimate and a capped period estimate.
Why your code might start with K
- You receive taxable state pension that is paid without tax deducted.
- You have benefits in kind (company car, private medical cover, etc.).
- You underpaid tax in an earlier tax year and HMRC is collecting it through PAYE.
- You have multiple income sources and your allowances are used elsewhere.
Example
If your monthly gross pay is £3,000 and your code is K500, the K element adds £5,000 taxable income per year. That increases your annual tax bill compared with a standard code because a larger portion of your income is taxed through payroll.
Important notes
- This is an educational estimator, not payroll software.
- Actual payslips can differ due to cumulative PAYE calculations, previous period adjustments, and tax code suffixes like M1/W1/X.
- Check your tax code notice in your personal tax account if numbers look wrong.