Every calculator on this site is tested against worked examples before it goes live, and the formula it uses is printed on the page. This page explains exactly what that means, so you can decide how much to trust a given result.
Where the formulas come from
We do not invent formulas. Each tool implements either a standard piece of mathematics (the Pythagorean theorem, compound interest, unit conversion factors defined by international standard) or a named, published method. Where a named method exists, the page says which one it is — for example Mifflin-St Jeor for basal metabolic rate, Devine for ideal body weight, Boer for lean body mass, Mosteller for body surface area, the Karvonen method for target heart rate, WCAG 2.1 relative luminance for colour contrast, and the standard amortisation formula for loan and mortgage payments.
Unit conversion factors use the internationally defined exact values where they exist — 1 inch = 25.4 mm, 1 pound = 0.45359237 kg, 1 US gallon = 3.785411784 L — rather than rounded approximations.
How results are verified
Each calculator ships with at least one worked example whose answer is known independently. Before publishing, the tool's code is executed against that example in an automated test run; if the output does not match, the tool is not published. Tools that produce random output — dice, pickers, password generators — are instead run repeatedly and checked for correct range, uniqueness and distribution rather than a fixed value.
The worked example is then printed on the page itself, so you can reproduce the check by hand.
What these tools cannot do
A calculator applies a formula to the numbers you give it. It does not know your circumstances. Financial tools ignore tax, fees, inflation and your personal situation unless a field explicitly asks for them. Health tools use population-level formulas that do not account for body composition, medical conditions, pregnancy, or athletic training. Treat every result as a starting point for a conversation with a professional, not a substitute for one.
Privacy of what you type
Calculations run entirely in your browser using JavaScript. The values you enter are never sent to a server, because there is no server to send them to — the site is static files. This applies to every tool, including the text, password and QR tools.
Updates and corrections
Each page carries a visible last-updated date. Pages are revisited when a standard changes, when a formula is superseded, or when a reader reports a problem. Corrections are prioritised over publishing new tools. Report anything that looks wrong to hello@calculorium.com and include the numbers you entered.