Why these tools exist, where their numbers come from and what happens before one of them is published.
A converter, a calculator and a generator are the same thing underneath: a function that turns input into output. Rather than building each tool as its own little application, this site describes each one as a definition — its inputs, its formula, the source that formula came from and the values it has to reproduce — and a shared engine renders it. That is why the tools behave consistently and why adding the next one is a matter of writing it up properly rather than writing more code.
Conversion factors are transcribed from the organisation that defines the quantity, and the citation stays next to the number on the page. Units come from NIST Special Publication 811, web formats from W3C specifications and MDN, and health measures from WHO fact sheets. Where a source stops, this site stops with it: the BMI tool shows only the two adult thresholds the WHO actually publishes rather than borrowing a third from somewhere else.
Every definition carries reference values taken from its source, and the build runs them on every change. A converter is also checked in both directions, at boundary values, and against exact rational arithmetic computed with integers rather than floating point. If a number stops reproducing, the build fails and the tool does not ship. There are currently 20 published tools and 98 reference values behind them.
That discipline exists because a converter that is quietly wrong is worse than no converter at all: nobody double-checks a number that looks plausible.
These tools have no backend. Values you type are converted by JavaScript running in your browser, and images are decoded and re-encoded by your browser's canvas. There is no upload step, no processing queue and no copy of your file on a server, which is also why the tools keep working if your connection drops after the page has loaded. What that means for advertising and logs is set out on the privacy page .
If a number here disagrees with the standard it cites, that is a defect worth reporting rather than a rounding preference. Corrections and requests for new tools are welcome.