# Kelvin

## SI unit of temperature / From Wikipedia, the free encyclopedia

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The **kelvin**, symbol **K**, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius.[1][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). The **Kelvin scale** is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its zero point.[2][5]

**Quick facts: Kelvin, General information, Unit system, Uni...**▼

Kelvin | |
---|---|

General information | |

Unit system | SI |

Unit of | temperature |

Symbol | K |

Named after | William Thomson, 1st Baron Kelvin |

Conversions | |

x K in ... | ... corresponds to ... |

Celsius | (x − 273.15) °C |

Fahrenheit | (1.8 x − 459.67) °F |

Rankine | 1.8 x °Ra |

Historically, the Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the melting point of ice to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the Boltzmann constant k to be exactly 1.380649×10^{−23} J⋅K^{−1}.[1] Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy *kT* by 1.380649×10^{−23} J. The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15,[2] meaning that a *change* or *difference* in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K.

The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius 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. Those are defined using the kelvin.[6][7]

According to SI convention, the kelvin is never referred to nor written as a *degree*. The word *kelvin* is not capitalised when used as a unit, but is pluralised as appropriate. The unit symbol K is a capital letter. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside".[8] It is common convention to capitalize Kelvin when referring to the Kelvin scale.[9]