# Kelvin

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The kelvin, symbol K, is a unit of measurement for temperature.[1] 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.[2] The kelvin is the base unit of temperature in the International System of Units (SI), used alongside its prefixed forms.[2][3][4] It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).[5]

Quick facts: kelvin, General information, Unit system, Uni...
kelvin
Thermometer with markings in degrees Celsius and in kelvins
General information
Unit systemSI
Unit oftemperature
SymbolK
Named afterWilliam Thomson, 1st Baron Kelvin
Conversions
x K in ...... corresponds to ...
Celsius   (x − 273.15) °C
Fahrenheit   (1.8x − 459.67) °F
Rankine   1.8x °Ra
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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.[1][5] This relationship remains accurate, but the Celsius, Fahrenheit, and Rankine scales are now defined in terms of the Kelvin scale.[2][6][7] 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.[5] In the United States, outside of the physical sciences, the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature.[6]

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