Unit Converter — Length, Weight, Temperature, Volume, Area and Speed

1. 1Choose a categorySelect length, weight, temperature, volume, area or speed. Each loads its most-searched pair by default — centimetres to inches, Celsius to Fahrenheit.
2. 2Enter a value and pick the unitsType the amount, then choose the unit to convert from and the one you want. The swap button reverses the direction.
3. 3Read the result and the factorThe converted value appears immediately with the factor used, and the table below lists common equivalences for the category.

Six categories and 39 units: length from millimetres to miles, weight from milligrams to tonnes and stones, temperature in Celsius, Fahrenheit and Kelvin, volume from millilitres to US and imperial gallons, area up to square miles, and speed in km/h, mph, m/s and knots.

Factors are the exact international definitions: an inch is exactly 0.0254 metres, a pound exactly 0.45359237 kilograms, a mile exactly 1609.344 metres. Using definitions rather than rounded values such as 2.54 means repeated conversions accumulate no error.

Most units convert by multiplication alone: 5 kilometres is 5 × 1000 metres. Temperature scales also have different zero points, so they need an offset as well as a factor. Celsius and Kelvin share a degree size but their zeros sit 273.15 apart, while Fahrenheit has a different zero and a degree five-ninths the size.

The conversions are °F = °C × 9/5 + 32, °C = (°F − 32) × 5/9, and K = °C + 273.15. The offset is why 0 °C is 32 °F. The scales coincide at exactly −40. Values below absolute zero — −273.15 °C, −459.67 °F, 0 K — are rejected.

Volume is where this causes real errors. A US gallon is 3.785411784 litres and an imperial gallon 4.54609 litres, making the imperial gallon about 20% larger; a US pint is 473 ml against an imperial pint's 568 ml. Both gallons are listed separately for that reason. Weight is safer, since the pound and ounce are identical in both systems, and the stone — 14 pounds, or 6.35 kg — remains in everyday use for body weight in the UK and Ireland. Recipes bite most often: a “cup” is 236.6 ml in the US but 250 ml in Australia.

Results show eight significant figures, keeping small values readable without discarding precision on large ones, and switch to scientific notation above 10¹⁵ or below 10⁻⁶. The arithmetic runs in double precision on the exact factors, so the display is a rounded result rather than a rounded calculation. Numbers follow your locale — 1,234.56 in English, 1.234,56 in German — and both decimal marks are accepted.

The factors ship with the page and the arithmetic runs in your browser, so no conversion involves a server request. Nothing you type is uploaded, logged or stored, and the page works with the network disconnected.