Kelvin (°K)

The kelvin is the base unit of thermodynamic temperature in the International System of Units. Its symbol is K, written without a degree sign and without the word degree, so temperatures are stated as 300 kelvins rather than 300 degrees kelvin.

Its zero is absolute zero, the temperature at which the thermal motion of particles reaches its quantum-mechanical minimum. Nothing can be colder, which makes the kelvin an absolute scale rather than a relative one. That property matters in physics because ratios become meaningful: a gas at 600 kelvins has twice the absolute temperature of one at 300, a statement that is simply false if the same temperatures are written in Celsius or Fahrenheit.

The size of the kelvin is identical to the size of the degree Celsius, so temperature differences are numerically the same in both. Only the offset differs, and it is exactly 273.15. William Thomson, later Lord Kelvin, proposed the absolute scale in 1848, reasoning from Carnot's work on heat engines that a thermodynamic temperature independent of any particular substance must exist.

Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 times ten to the power minus twenty-three joules per kelvin. The previous definition used the triple point of water, a real physical fixture that nonetheless depends on the isotopic composition of the sample. Cryogenics, astronomy, spectroscopy and colour temperature in photography and lighting all use the unit, which is why a warm white bulb is sold as 2700 K and daylight as 5500 K.

Whole disciplines live at particular points on the scale. The cosmic microwave background sits at 2.725 kelvin, the boiling point of liquid helium at 4.2, and superconductors of the older kind work below about 20, which is why they need helium rather than nitrogen. Liquid nitrogen boils at 77 kelvin and is cheap, so the discovery of materials superconducting above that mark in 1986 changed what was practical. At the other end, a filament lamp runs near 2800 kelvin, the surface of the Sun is 5800, and its core reaches fifteen million. Expressing all of this on one scale with a physical zero is what makes such comparisons meaningful.

Zero kelvin equals minus 273.15 degrees Celsius, and 273.15 kelvin equals 0 degrees Celsius.

Kelvin conversion table