Kelvin

The kelvin, symbol K, is a unit of measurement for temperature. The Kelvin scale is an absolute scale, which is defined such that 0 K is absolute zero and a change of thermodynamic temperature T by 1 kelvin corresponds to a change of thermal energy kT by 1.380649×10⁻²³ J. The Boltzmann constant k = 1.380649×10⁻²³ J⋅K⁻¹ was exactly defined in the 2019 redefinition of the SI base units such that the triple point of water is 273.16±0.0001 K. The kelvin is the base unit of temperature in the International System of Units (SI), used alongside its prefixed forms. It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).

Historically, the Kelvin scale was developed from the Celsius scale, such that 273.15 K was 0 °C (the approximate melting point of ice) and a change of one kelvin was exactly equal to a change of one degree Celsius. This relationship remains accurate, but the Celsius, Fahrenheit, and Rankine scales are now defined in terms of the Kelvin scale. The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries the Celsius scale remains the dominant scale outside of these fields. In the United States, outside of the physical sciences, the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature.