Kelvin (unit)

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The kelvin (symbol: K) is the SI unit increment of temperature and is one of the seven SI base units. The kelvin is defined as 1/273.16 of the thermodynamic temperature of the triple point of water[1]. Zero kelvin (0 K) is the thermodynamic absolute zero. In some disciplines, the term absolute temperature indicates the use of an absolute scale, such as Kelvin.

The kelvin is named after the Irish-born physicist and engineer William Thomson, 1st Baron Kelvin (1824 – 1907), who wrote of the need for an absolute thermometric scale.

The Celsius scale is related to the Kelvin (absolute) scale by setting the temperature zero degrees Celsius (0°C) to be exactly 273.15 K, and the increment of one degrees Celsius to be equal to one kelvin. The older Fahrenheit scale has 32 degrees Fahrenheit (32°F) equal to 273.15 K, and the increment of 1.8°F equal to one kelvin; thus absolute zero is -459.67°F.

Measurements made using any of the four major temperature scales (kelvin, Celsius, Fahrenheit and Rankine) can be readily converted. (See the Temperature conversion article for conversion equations.)


  1. International Bureau of Weights and Measures From the website of the International Bureau of Weights and Measures. The CIPM has specified the isotopic composition of standard water, as the triple point of water varies depending on the isotopic composition of water.