The SI unit of temperature commonly used in thermodynamics is the kelvin. Its symbol is K, and unlike Celsius or Fahrenheit, it is an absolute temperature scale whose zero is absolute zero.
A kelvin has the same size as one degree Celsius: a temperature increase of 1 K equals an increase of 1 °C. The scales differ in their zero points, however. To convert Celsius to kelvins, add 273.15; therefore, 0 °C equals 273.15 K, while water’s triple point is approximately 273.16 K under the historical definition.
The unit honors William Thomson, 1st Baron Kelvin, who proposed an absolute thermometric scale in 1848. The kelvin became an SI base unit in 1954. Since the 2019 SI revision, it has been defined by fixing the Boltzmann constant, linking temperature directly to microscopic energy.
Rankine is also an absolute temperature scale, but it is not the SI unit. Fahrenheit and Celsius are practical everyday scales, and “degrees Kelvin” is an outdated expression: the correct modern form is simply “kelvins,” such as 300 K.