degree Celsius °C

What degree Celsius means

The degree Celsius (°C) is a temperature scale defined so that water freezes at 0° and boils at 100° at standard atmospheric pressure (101.325 kPa, the average sea-level pressure on Earth). The size of one degree was set so that 100 degrees separate those two points — the scale is centigrade, literally "hundred steps." It is one of the most-used scales on the planet: every country that uses the metric system reports everyday temperature in °C, and almost all science and industry does too.

The scale was proposed in 1742 by the Swedish astronomer Anders Celsius, then a professor at Uppsala University. His original thermometer was inverted — zero marked the boiling point of water and 100 the freezing point. A year later, either Celsius himself or (more likely) his instrument maker Carl Linnaeus flipped the scale to the version we use today. The unit was originally called *centigrade*; in 1948 the 9th General Conference on Weights and Measures officially renamed it *degree Celsius* to avoid confusion with the centigrade angular unit.

Where Celsius is the everyday scale

Almost every country in the world uses Celsius for weather forecasts, cooking, and medical thermometers. The United States is the only major holdout: everyday American temperature is reported in Fahrenheit, although American science, medicine, and industry all quietly use Celsius internally. A few Caribbean nations (the Bahamas, Belize, the Cayman Islands) and a handful of Pacific states also default to Fahrenheit.

In science the Celsius scale is universal. Laboratory work, climate data, and engineering specifications are all written in °C or in kelvin (the absolute scale, which adds 273.15 to the Celsius reading). The International System of Units treats kelvin as the base unit but accepts Celsius for everyday use because its size is identical to one kelvin interval.

Common conversion reference points

A few temperature points come up so often that they are worth committing to memory:

- 0 °C = 32 °F — the freezing point of pure water at sea-level pressure. Also the point at which Celsius was originally defined.
- 20 °C = 68 °F — a comfortable indoor room temperature in most countries. This is roughly the standard "ambient" used in laboratory specifications.
- 37 °C = 98.6 °F — normal human body temperature, set in the 19th century from a single (slightly high) measurement and still the reference fever threshold.
- 100 °C = 212 °F — the boiling point of water at sea level. Drops by about 1 °C for every 285 m of altitude gained.
- −40 °C = −40 °F — the exact crossover point, the only temperature that reads identically on both scales. It is not a coincidence of rounding; it follows from the two scale definitions.

Why the scale was almost the other way around

Celsius's original 1742 paper described a scale running from 0 at the boiling point of water to 100 at the freezing point. The reasoning, as best historians can reconstruct, was to avoid negative numbers when describing the very cold winters of Uppsala. The reversal to the modern form happened quickly — within about a year — and is sometimes credited to Celsius himself and sometimes to his colleague Carl Linnaeus, who ran the botanical garden where the prototype thermometers lived. The 1743 instruments from the Linnaeus workshop are the earliest physical artifacts known to use the modern orientation.

The scale was redefined in 1954 by the 10th General Conference on Weights and Rates so that the triple point of water (the temperature at which ice, liquid water, and water vapour coexist in equilibrium) sits at exactly 0.01 °C, and the absolute zero sits at −273.15 °C. This locked the size of one degree to a fraction of the kelvin, which in turn is now defined by the Boltzmann constant rather than by any particular physical substance.

Frequently asked questions

**Why does −40 °C equal −40 °F?** Because the two scales cross at exactly that point. The Fahrenheit scale was defined in 1724 with 0 °F at the temperature of a brine solution (ice, water, and salt) and 96 °F as approximately human body temperature; the size of one Fahrenheit degree works out to 5/9 of a Celsius degree, and the offset puts the crossover exactly at −40.

**Why does science use kelvin instead of Celsius?** Because kelvin starts at absolute zero, the coldest temperature physically possible, and never goes negative. Many physical laws — gas laws, black-body radiation, the speed of sound — are simplest when written in absolute temperature. Celsius is fine for everyday life because the offset of 273.15 is irrelevant at human-scale temperatures.

**Is the degree Celsius part of SI?** Yes, but as a derived unit. The SI base unit is the kelvin; Celsius is officially the kelvin shifted by 273.15, which is why one degree is the same size on both scales.