Kelvin (K)

The kelvin is the SI base unit for temperature. It is currently defined by a fraction of the temperature of the triple point of water.

Accurate temperature measurement is widely important, from cooking to affecting the rate of chemical reactions.

In 1954, the kelvin scale was given its modern definition, using the triple point of water.

The triple point of water is the unique point where water can exist as a solid (ice), liquid and a gas (water vapour). These three states coexist in stable equilibrium at a certain temperature: 0.01 °C, or 273.16 K. One kelvin is currently defined as 1/273.16 of this temperature. However, this definition is not practical for extremely hot or cold temperatures.


It is expected that the kelvin will be redefined in May 2019 using the Boltzmann constant, a natural constant. This will link the kelvin to the other SI units in a stable manner, independent of any particular substance.