The SI unit of temperature is the kelvin, symbol K.

The degree Celsius, symbol °C, is the special name for the kelvin. For everyday use, the degree Celsius is the standard unit used to measure temperature.

Both the kelvin and the degree Celsius can refer either to a temperature difference, or to a distinct point on a temperature scale.

  • When used to refer to a temperature difference, the kelvin and the degree Celsius are the same size; the numerical value expressed in degrees Celsius is equal to the numerical value expressed in kelvins.
  • When used to refer to a point on a temperature scale, the kelvin and degree Celsius values are different. The Celsius and Kelvin scales have different zero points. The Kelvin scale has absolute zero as its zero point, whereas the Celsius scale has the freezing point of water as its zero point.
 Temperature kelvins degrees Celsius 
 boiling point of water 373.15 K 100 °C 
 normal body temperature 310.15 K 37 °C 
 freezing point of water 273.15 K 0 °C 
 absolute zero 0 K -273.15 °C 

The Celsius scale

The Celsius temperature scale was originally defined by two fixed points at standard atmospheric pressure: the freezing point of water, and the boiling point of water. The size of one degree Celsius was defined as 1100 of the difference in temperature between these two points.

The modern definition of the degree Celsius is more precise, but for most practical purposes 0 °C is the temperature at which water freezes, and 100 °C is the temperature at which water boils.

Celsius is the international standard temperature scale for weather reporting and weather forecasts.

desert heat 40 – 50 °C
extreme heat 35 – 40 °C
very hot 30 – 35 °C
hot 25 – 30 °C
warm 20 – 25 °C
mild 15 – 20 °C
cool 10 – 15 °C
chilly 5 – 10 °C
cold 0 – 5 °C
freezing cold -10 – 0 °C
bitter cold -20 – -10 °C
extreme cold -30 – -20 °C

The degree Celsius is used in central heating thermostats, domestic ovens, refrigeration equipment and clinical thermometers.


Note that the symbol for degree Celsius should always include the ° sign.
i.e. °C, and never just C.