Kelvin

kelvin
Equivalent temperatures in kelvin (K), Celsius (°C), and Fahrenheit (°F)
General information
Unit system	SI
Unit of	temperature
Symbol	K
Named after	William Thomson, 1st Baron Kelvin
2019 definition	kB ≝ 1.380649×10−23 J/K
Conversions
x K in ...	... corresponds to ...
Celsius	(x − 273.15) °C
Fahrenheit	(1.8 x − 459.67) °F
Rankine	1.8 x °Ra

The kelvin, symbol K, is the base unit of measurement for temperature in the International System of Units (SI).^[1][2][3][4] It is named after the 19^th century British scientist William Thomson, titled Lord Kelvin.^[5] Thomson proposed an absolute temperature scale (now named the Kelvin scale) that keeps the same temperature increment used in the well-established Celsius scale (symbol °C) while setting the coldest possible temperature (now defined as -273.15 °C) to 0 K. Thus, a Celsius temperature is converted to kelvin by adding 273.15. Although the Celsius, Fahrenheit, and Rankine scales are now defined in terms of the Kelvin scale, this conversion still applies.^[2][6][7] For example, 0 °C, the approximate melting point of ice, is 273.15 K.^[1][5]

Precise measurements of water's triple point resulted in the 1954 adoption of the triple point as the kelvin's reference. But because the triple point still has some measurement uncertainty, the 2019 redefinition of the SI base units now defines the kelvin by setting the Boltzmann constant to exactly 1.380649×10⁻²³ joules per kelvin, thereby removing any uncertainty from the Boltzmann constant.^[2] Multiplying the Boltzmann constant by an exact temperature now yields an exact energy, so every 1 K change of thermodynamic temperature corresponds to a thermal energy change of exactly 1.380649×10⁻²³ J.