Celsius

degree Celsius
A thermometer calibrated in degrees Celsius, showing a temperature of −17 °C
General information
Unit system	SI
Unit of	temperature
Symbol	°C
Named after	Anders Celsius
Conversions
x °C in ...	... corresponds to ...
SI base units	(x + 273.15) K
Imperial/US units	(⁠9/5⁠x + 32) °F

The degree Celsius is the unit of temperature on the Celsius temperature scale^[1] (originally known as the centigrade scale outside Sweden),^[2] one of two temperature scales used in the International System of Units (SI), the other being the closely related Kelvin scale. The degree Celsius (symbol: °C) can refer to a specific point on the Celsius temperature scale or to a difference or range between two temperatures. It is named after the Swedish astronomer Anders Celsius (1701–1744), who proposed the first version of it in 1742. The unit was called centigrade in several languages (from the Latin centum, which means 100, and gradus, which means steps) for many years. In 1948, the International Committee for Weights and Measures^[3] renamed it to honor Celsius and also to remove confusion with the term for one hundredth of a gradian in some languages. Most countries use this scale (the Fahrenheit scale is still used in the United States, some island territories, and Liberia).

Throughout the 19th century, the scale was based on 0 °C for the freezing point of water and 100 °C for the boiling point of water at 1 atm pressure. (In Celsius's initial proposal, the values were reversed: the boiling point was 0 degrees and the freezing point was 100 degrees.)

Between 1954 and 2019, the precise definitions of the unit degree Celsius and the Celsius temperature scale used absolute zero and the triple point of water. Since 2007, the Celsius temperature scale has been defined in terms of the kelvin, the SI base unit of thermodynamic temperature (symbol: K). Absolute zero, the lowest temperature, is now defined as being exactly 0 K and −273.15 °C.^[4]