Length is where the metric and imperial worlds meet most often, and where the numbers are least negotiable: since the 1959 international yard and pound agreement, an inch is exactly 25.4 millimetres and a mile exactly 1609.344 metres. Those are definitions rather than measurements, so every converter here divides by the exact factor instead of multiplying by a rounded reciprocal.
Convert kilometers to miles instantly in your browser. One mile is exactly 1.609344 kilometers, the factor fixed by the international yard and pound agreement.
Convert miles to kilometers in your browser. One mile is exactly 1.609344 kilometers, so the answer is a fixed ratio rather than an approximation.
Convert meters to feet instantly. A foot is exactly 0.3048 meters, so the conversion is a definition rather than a measurement.
Convert centimeters to inches in your browser. An inch is exactly 25.4 millimeters, fixed by international agreement in 1959.
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What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
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Every tool in this category runs entirely in your browser: there is no upload, no account and no request to a server once the page has loaded. The numbers behind them are transcribed from the organisation that defines them — NIST for units, the W3C for web formats, the WHO for health measures — and each page names its source next to the formula. A tool whose reference values stop reproducing fails the build and does not ship, which is the whole reason the catalogue grows slowly.