Tax Code K100 Explained
K code — £1,000 is added to your taxable income because your untaxed benefits or arrears exceed your tax-free allowance.
At a glance — K100
- Personal Allowance
- −£1,000 (addition to taxable income)
- Region
- UK-wide (rUK / Wales)
- Cumulative?
- Yes
- Code type
- K code (addition)
What does K100 mean?
You have income that is not being taxed another way and it is worth more than your tax-free allowance. The number is added to your taxable income rather than deducted.
Your taxable income is increased by £1,000 to account for untaxed benefits.
How much tax will you pay on K100?
Annual income tax HMRC would deduct on 2025-26 rates for a range of salaries, assuming the full year on this code. National Insurance is additional and the same across the UK.
| Annual salary | England / Wales / NI — income tax | Effective rate | Take-home (pre-NI) |
|---|---|---|---|
| £20,000 | £4,200 | 21.0% | £15,800 |
| £30,000 | £6,200 | 20.7% | £23,800 |
| £50,000 | £12,860 | 25.7% | £37,140 |
| £80,000 | £24,860 | 31.1% | £55,140 |
| £100,000 | £32,860 | 32.9% | £67,140 |
For Scottish / rUK side-by-side, or to model your own salary, student loan and pension contributions, use the tax code checker.
What you'll actually pay vs the standard code
K100 compared with 1257L — the standard code most single-job employees with no adjustments are on — at the same salary, 2025-26 rates.
| Annual salary | K100 tax | 1257L tax | Difference |
|---|---|---|---|
| £30,000 | £6,200 | £3,486 | +£2,714 |
| £50,000 | £12,860 | £7,486 | +£5,374 |
| £80,000 | £24,860 | £19,432 | +£5,428 |
Should you be on tax code K100?
K codes are correct when HMRC needs to collect more tax than your Personal Allowance shelters — typically company car benefit, medical insurance, state pension or underpaid tax from a previous year. If the code looks high relative to your benefits in kind, request a P2 coding notice from HMRC and check the detail.
How to fix it if K100 is wrong
- Check your current code in the HMRC app, your Personal Tax Account, or your latest P2 coding notice or payslip.
- New job with no P45? Ask your employer for a Starter Checklist — completing it correctly is what moves you off a temporary code.
- Not sure what your P45 figures mean, or already overpaid? See the P45 box-by-box guide to decode what your employer reported, or run the tax refund estimator to check what you might be owed.
- If HMRC agrees your code was wrong, most in-year corrections refund automatically through payroll; larger over- or underpayments are settled via a P800 calculation after the tax year ends.
Related tax codes
K200 — K Code £2,000 Addition
K code — £2,000 is added to your taxable income (typically reflecting company car benefit, medical insurance or underpaid tax).
K274 — K Code £2,740 Addition
K code — £2,740 is added to your taxable income each year, usually because benefits in kind (e.g. company car) outweigh your Personal Allowance.
K475 — K Code £4,750 Addition
K code — £4,750 is added to your taxable income. Common for employees with a mid-range company car or modest employer benefits.
K500 — K Code £5,000 Addition
K code — £5,000 is added to your taxable income each year, commonly from company car BIK or state pension exceeding the Personal Allowance.
Check your own code: enter any HMRC tax code into the free tax code checker — it decodes the letters and number, shows your Personal Allowance, and estimates your take-home using 2025-26 or 2026-27 rates.
For the full plain-English guide to every UK tax code letter, prefix and suffix, see UK Tax Codes Explained 2026-27.
Sources
Tax code rules from gov.uk/tax-codes. Income tax rates from HMRC. Effective tax figures computed live from central configuration — correct for 2025-26.