Kelvin (K)
Definition
The SI base unit of temperature, fixed since 2019 by defining the Boltzmann constant as exactly 1.380649×10⁻²³ J/K — 0 K is absolute zero.
History
The kelvin is named after William Thomson, Lord Kelvin, who proposed an absolute temperature scale in 1848 starting from absolute zero. Since 2019 it has been defined by fixing the Boltzmann constant, tying temperature directly to energy.
Where it is used
The kelvin is the scientific temperature unit used in physics and engineering, and also describes the colour temperature of light sources.
Typical uses include science and physics, colour temperature.
Conversion pages
These pages convert kelvin to or from another unit in temperature.
Other Temperature units
Sources
- BIPM SI BIPM, The International System of Units (SI), 9th ed. (2019) · Dataset reviewed Nov 11, 2026