How we work and our sources

Our aim is that every result can be checked. These are the steps every tool goes through before it is published:

1. Official source: we identify the law, article or official body that governs the calculation. We don't publish a tool if we can't name its source.
2. Rule in plain words: we write down the rule and formula exactly as shown on the page.
3. Testing: we write at least five hand-worked cases and make sure the calculator gives the same result every time.
4. Review: a person checks the rule against the official text, and the review date is shown on the page.
5. Follow-up: we watch for changes to laws and rates, and update the tool and its date when they change.

Calculations run in your browser and we don't store what you enter. If you find a mistake, email us and we'll look into it.

Tools and their sources

Spot a law update? Let us know →