Calculation Methodology

1. Choosing the formula

Each calculator starts from a written specification that names the formula, the method and its sources. Formulas come from standard references such as open textbooks, standards bodies and official publications. When there are several accepted methods, the page says which one it uses and why it matters. When no authoritative definition exists, the page states the definition it uses.

2. Units and conversions

Unit conversions go through a base unit, using exact defined factors where they exist, for example the international inch of 25.4 millimetres. Pages show which units they accept, and a result is labelled with its unit.

3. Input validation

Every field has an allowed range stated on the page. Empty, non-numeric or out-of-range input produces a message next to the field and no result, instead of a misleading number.

4. Precision and rounding

Calculations use double-precision floating point. Results are rounded only for display, to the number of decimals stated on the page, and money amounts use the rounding rule the page states.

5. Worked examples

The worked examples on a page are produced by running the calculator with the inputs shown. A validation step fails the build if an example no longer matches the calculator.

6. Boundary conditions

The specification lists edge cases, such as zero, the ends of a range, a leap day or a division by zero, and the tests include them together with invalid input.

7. Automated testing

For each calculator, the expected values are produced separately by a program written in a different language, then compared with the website implementation. Across the site this is more than four thousand six hundred checks for over a thousand calculator specifications, and they all pass at the time of writing. Passing the checks shows that the code matches its specification. It does not prove the specification is the right method for your situation.

8. Source verification

Each page lists its sources. Figures that change by year or place, such as tax limits, are kept with their year, effective date, issuing authority and the date they were last compared with it. Citations are at title and publisher level unless a page says otherwise. See sources and references.

9. Limitations

• Checking is not professional review. Calculators in health, tax, finance, engineering, food safety and similar areas are held back from public release until that review has happened.
• Calculators give a general estimate and do not know your personal circumstances.
• Rules change; a figure may be out of date before we update it.

Related: editorial policy, corrections policy, disclaimer.