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How the numbers are checked

Every figure on this site starts in the rule that sets it: a statute, a tax authority's guidance, a regulator's handbook or, for the word tools, an open dictionary. It is stored once in the tool's data file, and the calculator, the examples and the text all read from there. Automated tests check the arithmetic against known answers before anything is published, and each page dates its last check and lists its sources.

1. 1

   Start from the law

   Each figure is taken from the statute, tax authority or regulator that sets it, and each word list from an open dictionary named on the page, never from another tool.

2. 2

   Keep every figure in one place

   Rates, limits and word lists live in data files for each tool, so the tool, the worked examples and the text cannot disagree.

3. 3

   Test the arithmetic

   Automated tests compare every formula with reference values, and a page that fails them is never published.

4. 4

   Date every check

   Each page shows when its figures were last verified and lists its sources. Reported errors are corrected and the date updated.