Methodology

1. Rules come from official sources

Every rate, threshold and allowance is taken from the tax authority or legislation for that country — for the UK, GOV.UK, HMRC, Revenue Scotland, the Welsh Revenue Authority and legislation.gov.uk. Each calculator lists the exact pages it relies on, and our UK source registry shows them all with review dates. We do not use other calculators as a primary source.

2. Rules are versioned by tax year

Rules live in a separate calculation engine, outside the page design, in one package per country and tax year (for example uk/2026-27). When a new tax year is announced we copy the previous year as a starting point and edit the copy — earlier years are never overwritten. The current UK rule version is uk-2026-27.1, reviewed on 2026-10-07.

3. Every rule is tested before release

• Boundary tests at exact thresholds and one pound either side — for example £12,569, £12,570 and £12,571, £100,000 and £125,140 for the Personal Allowance taper.
• Worked examples checked by hand against official guidance, such as SDLT on £300,000 or Corporation Tax marginal relief on £100,000.
• Interface tests on desktop and mobile to confirm results render, inputs are labelled and nothing overflows small screens.

Worked examples on our pages are generated by the same engine as the calculator, so the text cannot drift from the results.

4. Results show their working

Each result shows the formula, a line-by-line breakdown, the bands used, effective and marginal rates, and the assumptions behind the estimate. Where real payroll or tax returns can differ — such as National Insurance being assessed per pay period — we say so next to the result.

5. Scope and limits

Calculators cover the common cases for individuals and small companies. Complex situations — non-residence for income tax, trusts, foreign income, detailed tax-code histories, losses carried across years — are flagged as out of scope rather than guessed. Results are estimates, not personalised advice.

6. Corrections

If you think a result is wrong, please contact us with the inputs you used. Confirmed errors are fixed, tested and released with a new rule version.

7. Independence from advertising

Calculator results never depend on advertising or affiliate relationships. Ads and any clearly labelled partner offers are placed outside the input and result area.