How this calculator is tested
Anyone can claim a tax calculator is accurate. This page shows the working, so you can decide for yourself. It covers what has been checked, what the checks produced, and — just as importantly — what this estimator does not handle.
How the engine is tested
The calculator is checked against real HMRC tax calculations: actual SA302 documents produced by HMRC from filed Self Assessment returns. For each one, the taxpayer's income is fed into this engine and the output is compared with what HMRC itself worked out, line by line — Income Tax, Class 2 and Class 4 National Insurance, and the final total.
A check only passes if the figure matches to within half a penny. The tests run automatically every time the calculator changes, and the site is not published if any of them fail.
The source documents are not reproduced here. They contain real people's names and tax reference numbers, so they are referenced by sample number only.
The checks, in full
These are the actual expected values in the test suite, not illustrations.
| Tax year | Self-employment profit | Income Tax | National Insurance | Total due |
|---|---|---|---|---|
| 2022-23 | £139,500 | £48,260.00 | £6,332.39 | £54,592.39 |
| 2023-24 | £118,900 | £38,772.00 | £4,945.00 | £43,717.00 |
| 2024-25 | £142,850 | £50,485.50 | £4,113.60 | £54,599.10 |
Each of those totals is matched exactly by this calculator.
A few details worth drawing out, because they show the engine handles the awkward parts rather than just the simple ones:
- The £100,000 Personal Allowance taper works. In the 2023-24 case, a profit of £118,900 tapers the Personal Allowance down to exactly £3,120, and the engine produces that figure independently.
- Each year uses its own rules, not this year's. The 2022-23 case exercises that year's blended Class 4 National Insurance rates, which existed only in 2022-23 because the rate changed mid-year.
- Class 2 is handled correctly across the reform. It is charged in the 2022-23 case (£163.80) and correctly comes out at £0 for 2024-25, where it is treated as paid rather than charged.
- National Insurance is rounded down per band, which is what HMRC does. Rounding the ordinary way puts the total out by pennies.
What has not been verified against a real calculation
2025-26 has not been checked against a real HMRC document, because it cannot be yet. Returns for that year aren't due until 31 January 2027, so no SA302 exists for it. The 2025-26 figures come from HMRC's published rates, bands and thresholds, and follow the same calculation order as the verified years — but that is a different and weaker kind of assurance, and you should treat it accordingly.
What this estimator does not cover
A general estimator cannot cover every situation, and pretending otherwise would be worse than useless. This one does not handle:
- Scottish or Welsh rates of Income Tax. Every figure uses England and Northern Ireland rates. If you are a Scottish taxpayer, the Income Tax result will be wrong for you.
- Student loan and postgraduate loan repayments.
- The High Income Child Benefit Charge.
- Marriage Allowance, Blind Person's Allowance and other transferable allowances.
- Capital Gains Tax, Gift Aid relief, and payments on account.
- Any adjustment carried over from a previous year, such as underpaid tax collected through your code.
If your situation includes any of these, the number here will be an incomplete picture rather than a wrong one — but it will still be off.
Where the rates come from
Bands, allowances and National Insurance thresholds for each supported year are taken from HMRC's published rates and the relevant Finance Act, and each year is stored as its own separate dataset. Nothing is carried over from one year to the next by assumption, which is the most common way a tax calculator quietly goes wrong: it keeps using last year's thresholds after April.
Found something wrong?
Please tell us, ideally with the figures you entered and what you expected. A reproducible correction is genuinely the most valuable thing anyone sends this site, and errors get fixed and re-tested rather than argued with.
This is still an estimate. It is not an official HMRC document and cannot be sent to a mortgage lender in place of a real SA302. When the exact figure matters, confirm it with HMRC or a qualified accountant.