This free online unit converter covers Length, Weight, Temperature, Area, and Volume, with an extensive range of units from everyday metric and imperial measurements to scientific and astronomical scales. Choose a category, pick your units, enter a value, and get an instant, precise conversion.
Each category has one base unit, and every other unit in that category is stored as a multiple of it. A conversion multiplies your value by the size of the "From" unit to reach the base, then divides by the size of the "To" unit. Switching categories reloads the unit lists, because only units that measure the same quantity can be converted into one another.
| Category | Base unit | Range of units |
|---|---|---|
| Length | metre | attometre to parsec, plus inch, foot, yard, mile and fathom |
| Weight | kilogram | atomic mass unit to solar mass, plus quintal, ounce, pound, stone and carat |
| Area | square metre | square millimetre to square mile, plus are, hectare and acre |
| Volume | litre | millilitre to cubic metre, US and UK gallons, US fluid ounce and oil barrel |
| Temperature | Celsius (as a reference) | Celsius, Fahrenheit, Kelvin, Rankine and Réaumur |
Imperial and US customary units use their exact legal definitions, such as 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 US gallon = 3.785411784 L and 1 UK gallon = 4.54609 L. The astronomical unit is also exact (149,597,870,700 m, fixed by the International Astronomical Union in 2012), and the light year here is the distance light travels in a Julian year of 365.25 days.
The result is shown with up to eight decimal places. That is plenty for everyday conversions, but it means an extremely small answer can appear as 0. The Bohr radius, for example, is about 0.0000000000529 metres, which rounds to zero at eight decimals. When that happens, pick a smaller target unit: the same length is about 52.92 picometres.
Very large answers are shown in full with digit grouping, which can be hard to read. Converting the mass of the Sun to kilograms produces a 31-digit number. Choosing a larger target unit, such as Earth masses (the Sun is about 333,054 Earth masses), gives a figure that is easier to compare.
Keep in mind that a few units are measured quantities rather than definitions. Earth mass, solar mass and the radius of the Sun are stored as rounded scientific estimates, so results using them are accurate to about four significant figures, not to the last digit shown.
Rankine is to Fahrenheit what Kelvin is to Celsius: it starts at absolute zero but uses Fahrenheit-sized degrees, so 0 °R is absolute zero and 491.67 °R is the freezing point of water. It still turns up in some American engineering tables. Réaumur divides the gap between freezing and boiling water into 80 degrees instead of 100, so 37 °C is 29.6 °Ré. It is mainly met in old European recipes and historical documents.