Celsius (°C)

The degree Celsius is a unit of temperature on a scale where water freezes at 0 and boils at 100 under standard atmospheric pressure. Its symbol is the degree sign followed by C. It is the everyday temperature unit of almost every country in the world, and its degree is exactly the same size as the kelvin.

Anders Celsius proposed the scale in 1742 with the numbers the other way round: 0 for the boiling point of water and 100 for its freezing point. Whether Celsius himself, Carl Linnaeus or the instrument maker Daniel Ekström reversed it is disputed, but the inversion happened within a few years of his death and the scale has run upward ever since. It was called the centigrade scale for two centuries and renamed after Celsius in 1948, partly because centigrade already meant a hundredth of a right angle in French and Spanish surveying.

The modern definition no longer refers to water at all. The Celsius scale is defined from the kelvin, with zero degrees Celsius set at exactly 273.15 kelvin, and the kelvin itself fixed by the Boltzmann constant since 2019. The freezing and boiling points of water are now measured results rather than definitions, and neither falls at exactly 0 or 100 under all conditions.

The United States is the only major country that reports weather in Fahrenheit, and even there the sciences work in Celsius. Body temperature, cooking, weather and industrial process control all use the unit, and its two anchor points are memorable enough that most people can estimate an unfamiliar temperature without conversion.

A distinction of grammar carries a real meaning here. A temperature is written as a degree Celsius, but a difference between two temperatures is written as a kelvin, because the two scales share the same interval and differ only in where zero sits. Heat calculations therefore express specific heat capacity in joules per kilogram per kelvin even when the temperatures themselves are quoted in Celsius, and a rise of five degrees Celsius is a rise of five kelvin exactly. The convention looks pedantic until a calculation mixes an absolute temperature with a temperature difference, at which point keeping them apart is the only thing that prevents an offset of 273.15 from entering the arithmetic.

Zero degrees Celsius equals 273.15 kelvin or 32 degrees Fahrenheit, and a change of one degree Celsius equals a change of 1.8 degrees Fahrenheit.

Celsius conversion table