Kelvin (K)
Definition
The kelvin (symbol K) is the base unit of thermodynamic temperature in the International System of Units (SI). It is defined by fixing the numerical value of the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin and uses absolute zero as its origin.
History
The Irish-Scottish physicist William Thomson, Lord Kelvin, proposed an absolute temperature scale in 1848 anchored at absolute zero, the theoretical limit at which thermal motion ceases. The scale was originally defined to share its degree size with the Celsius scale.
From 1954 to 2019 the kelvin was defined so that the triple point of water occurred at exactly 273.16 K. The 2019 SI revision redefined it instead by fixing the Boltzmann constant, decoupling the unit from the properties of any specific substance. The change preserved the size of the kelvin to within the experimental uncertainty.
Standard reference
Defined by the BIPM SI Brochure (9th edition, 2019) by fixing the Boltzmann constant k at 1.380649 × 10⁻²³ J/K exactly.
Common conversions
| 273.15 K | = 0 °C |
|---|---|
| = 32 °F | |
| = 491.67 °R |