About this converter

319 units across 34 categories, with 786 conversion English pages with conversion formulas, worked examples and a reference table. Conversion happens as you type — there is no button to press and nothing to wait for.

Category and conversion pages link the definitions and conventions used for their units. References include the BIPM SI Brochure, NIST Special Publication 811 for customary factors, and NIST guidance on binary prefixes for digital units.

Most of these are definitions, not measurements. An inch is exactly 2.54 centimetres and a pound is exactly 0.45359237 kilograms. Other values are conventional or approximate, such as Mach at a stated atmospheric condition and measured atomic mass. Calculations use JavaScript floating point arithmetic, which has finite precision. Displayed formulas and results may be rounded; the precision control changes the displayed result without changing the underlying calculation.

Several widely used units have no single universal value, and this converter says so on the page rather than quietly picking one. A month and a year are averages of the Gregorian calendar. Mach depends on air temperature, so the sea-level ICAO Standard Atmosphere value is used. A gallon, a cup and a tablespoon differ between the US, imperial and metric systems, so each is a separate unit with its system in its name.

Some conversions are refused outright because they are not unit conversions at all. Candela, lumen and lux measure three different quantities; converting between them needs a solid angle or an area. Gray and sievert share a dimension but require a radiation weighting factor. Rather than invent a factor, the converter keeps these in separate categories and explains why.

Free to use, with no account and nothing to install. Nothing you type is sent anywhere — every conversion runs in your browser — and your favourites, recent conversions and theme choice are stored on your own device and never leave it. Advertising and analytics from Google keep the site running and do set cookies; the privacy policy explains exactly what they collect and how to opt out.