# Kelvin

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Kelvin temperature conversion formulas
Conversion from to Formula
kelvins degrees Celsius °C = K − 273.15
degrees Celsius kelvins K = °C + 273.15
kelvins degrees Fahrenheit °F = K × 1.8 − 459.67
degrees Fahrenheit kelvins K = (°F + 459.67) / 1.8
Note that for temperature intervals rather than temperature readings,
1 K = 1 °C and 1 K = 1.8 °F
Conversion calculator for units of temperature

The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. It is defined as the fraction 1/273.16 of the thermodynamic (absolute) temperature of the triple point of water.

A temperature given in kelvins, without further qualification, is measured with respect to absolute zero, where molecular motion stops (except for the residual quantum mechanical zero-point energy). It is also common to give a temperature relative to the Celsius temperature scale, with a reference temperature of 0° C = 273.15 K, approximately the melting point of water under ordinary conditions.

The kelvin is named after the British physicist and engineer William Thomson, 1st Baron Kelvin; his barony was in turn named after the River Kelvin, which runs through the grounds of the University of Glasgow.

## Typographical conventions Edit

The word kelvin as an SI unit is correctly written with a lowercase k (unless at the beginning of a sentence), and is never preceded by the words degree or degrees, or the symbol °, unlike degrees Fahrenheit, or degrees Celsius. This is because the latter are adjectives, whereas kelvin is a noun. It takes the normal plural form by adding an s in English: kelvins. When the kelvin was introduced in 1954 (10th General Conference on Weights and Measures (CGPM), Resolution 3, CR 79), it was the "degree Kelvin", and written °K; the "degree" was dropped in 1967 (13th CGPM, Resolution 3, CR 104).

Note that the symbol for the kelvin unit is always a capital K and never italicised. There is a space between the number and the K, as with all other SI units.

Unicode includes the "kelvin sign" at U+212A (in your browser it looks like K). However, the "kelvin sign" is canonically decomposed into U+004B, thereby seen as a (preexisting) encoding mistake, and it is better to use U+004B (K) directly.

## Conversion factors Edit

### Kelvins and Celsius Edit

The Celsius temperature scale is now defined in terms of the kelvin, with 0 °C corresponding to 273.15 kelvins.

• kelvins to degrees Celsius
$\mathrm{C} = \mathrm{K} - 273.15$

## Temperature and energy Edit

Strictly speaking, the temperature of a system is well-defined only if its particles (atoms, molecules, electrons) are at equilibrium and obey a Boltzmann distribution (or its quantum mechanical counterpart). In a thermodynamic system, the energy of the particles of a perfect gas is proportional to the absolute temperature, where the constant of proportionality is the Boltzmann constant. As a result, it is possible to determine the average kinetic energy $\overline{\mathrm{E_{kin}}}$ of the gas particles at the temperature T, or to calculate the temperature of the gas from the average kinetic energy of the particles:

$\overline{\mathrm{E_{kin}}} = \frac{3}{2} \cdot k_B \cdot \mathrm{T}$

The temperature of equilibrium electromagnetic radiation, a system of photons, is determined by the energy intensity, as given by Planck's blackbody distribution law.