How we calculate

Formulas from primary sources, tested three ways, dated on every page.

Formulas come from primary sources

Every calculator uses a published formula or a published data table, and the page names it: the Mifflin-St Jeor equation for calories, the standard amortisation formula for loans, the American Academy of Pediatrics dosing table for children's medicine, and so on. The sources are listed at the bottom of each page with links. When a topic has competing formulas we say which one we use and why, and often show the alternatives in a comparison table.

Some calculators depend on reference data that changes over time: tax bands, price benchmarks, exchange rates, device specifications. Those pages show a "data as of" date so you know how fresh the numbers are.

Three layers of testing

1. Formula tests. Each calculator has worked examples with known answers, taken from the source or computed by hand. The code must reproduce them exactly before a page can be published. The same examples appear on the page under "Example" so you can check them yourself.
2. Browser tests. Every page is opened automatically on a desktop and a phone-sized screen. We check that the result appears without scrolling, that every field works, that warnings fire on the documented edge cases, that nothing overflows, and that no errors occur.
3. Human review. Before launch and after any change, a person opens the calculator, tries realistic and unrealistic inputs, and reads the explanation for anything misleading.

Dates on every page

At the bottom of each calculator is a short verification history: when the formula and texts were last checked, the date of any reference data, and, where the page lists emergency phone numbers, when those numbers were last confirmed. If a date looks old to you, treat the page with more caution and tell us.

Emergency numbers

A few calculators, such as the ones for pet poisoning or medicine doses, show who to call when a result is dangerous. The numbers are kept in one directory, matched to your country, and checked against the official websites of the services rather than copied from other sites. Each entry carries the date of its last check, and our build fails if any entry goes unchecked for more than a year.

What we do not do

• We do not round results to look precise. Decimals shown match the precision the formula supports.
• We do not hide assumptions. If a calculator assumes a 14-day luteal phase or a 45% platform cut, the page says so and usually lets you change it.
• We do not give personal medical, legal or financial advice. Calculators estimate; a professional decides.

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