What Is a Kelvin? The SI Unit of Temperature

The kelvin (symbol K) is the SI base unit of thermodynamic temperature. It is one of the seven base units of the International System of Units, together with the second, meter, kilogram, ampere, mole and candela.

Since the 2019 revision of the SI, the kelvin is defined by fixing the numerical value of the Boltzmann constant k at exactly 1.380649 × 10⁻²³ J/K. The Boltzmann constant links temperature to energy: a change of one kelvin in temperature T corresponds to a change of exactly 1.380649 × 10⁻²³ joule in the energy kT.

Before 2019, the kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water. The new definition keeps the size of the unit the same but no longer depends on the properties of a particular water sample.

The kelvin scale starts at absolute zero, 0 K, the lowest possible temperature. Absolute zero is −273.15 °C and −459.67 °F.

Because the scale starts at absolute zero, there are no negative kelvin temperatures in ordinary use, and ratios are meaningful: 600 K is twice the thermodynamic temperature of 300 K. That is not true of Celsius or Fahrenheit values.

A kelvin and a degree Celsius are exactly the same size. Only the starting point differs: the Celsius scale is the kelvin scale shifted by exactly 273.15.

• °C = K − 273.15
• K = °C + 273.15

So 0 °C is 273.15 K, and 25 °C is 298.15 K. For a temperature difference there is no offset at all: a rise of 1 K is a rise of 1 °C.

To convert from kelvins to degrees Fahrenheit, multiply by 1.8 and subtract 459.67. For example, 300 K is 80.33 °F.

The unit was once called the "degree Kelvin" with the symbol °K. The 13th General Conference on Weights and Measures (CGPM), in 1967/68, renamed it the kelvin with the symbol K. The kelvin is now a unit in its own right, like the meter, not a degree on a scale.

• Write 300 K, not 300 °K and not 300 degrees kelvin.
• The unit name is lowercase (kelvin) and the plural is kelvins. The symbol is an uppercase K.
• Leave a space between the number and the symbol: 273.15 K, as with other SI units.
• Degree Celsius keeps its degree sign: 20 °C.

The Rankine scale (°R) is the absolute scale that uses Fahrenheit-sized degrees. Like the kelvin scale it starts at absolute zero, so 0 °R = 0 K. One degree Rankine is 5/9 of a kelvin, and 0 °F is exactly 459.67 °R. A temperature of 300 K is 540 °R.

Rankine appears mainly in older US engineering thermodynamics, where calculations need an absolute temperature but the input data are in degrees Fahrenheit.

Kelvins are the standard unit in physics, chemistry and engineering, especially in any formula that uses absolute temperature, such as the ideal gas law or the Stefan-Boltzmann law. Cryogenics, astronomy and materials science report temperatures in kelvins.

Many everyday products also use the kelvin. Lamp packaging, for example, gives the color temperature of the light in kelvins. Temperature coefficients of electronic parts are often given per kelvin, which is the same as per degree Celsius.