How our calculators work

Every PayPrompt calculator follows the same method, so you can see where each number comes from and what it does not cover.

Sources

Rates, thresholds and rules come from official sources such as GOV.UK, HMRC and the scheme or body that runs each scheme. Each calculator lists the pages it uses and the date they were checked. Rules were last checked 13 October 2026.

One place for each rule

Each rate is stored once in the calculation code, and the page text and tables are generated from it. When a rate changes it is updated in one place.

Testing

Each calculator has automated tests that check its results against worked examples, including those published by government, and against the reference tables shown on the page.

Rounding

Calculations keep full precision and round only for display, unless an official rule says to round at a particular step. Where that happens, the calculator says so.

Scope and limits

Each calculator states which nations and situations it covers, and which it does not. Results are estimates, not advice. Read our editorial policy or browse the UK calculators.