How We Test Our Calculators

A calculator is only useful if you can trust the answer. This is the process every calculator on this site goes through before it is published, and how we keep it accurate afterwards.

1. Start from a published reference

Each formula comes from a source we can cite: manufacturer specifications (for example, the printed yield of a bag of concrete), building and health codes, government agencies, standards bodies, or university extension services. Where sources disagree, we say so on the page and explain which figure we use and why.

2. Test against worked examples

Every calculator has automated tests that run its math against reference answers worked out by hand or taken from the source. Tests run each time the site is built. If any result drifts, the update is blocked until it is fixed.

3. Exact unit conversions

Conversions use exact definitions: 1 foot is 0.3048 metres, 1 US gallon is 3.785411784 litres, 1 pound is 0.45359237 kilograms. Results are rounded only for display, never in the middle of a calculation.

4. Show the working

Each page explains the formula, includes a worked example and a reference table, and lists its assumptions. If a result depends on local rules, such as septic tank minimums or building codes, the page says so plainly.

5. Keep it current

Pages show the date they were last updated. Figures that change on a schedule, such as tax brackets, benefit rates and mileage rates, are updated when the official numbers are published.

Found an error?

Please tell us. We check every report, fix confirmed errors promptly, and update the page's date.