What is the Kelvin scale?
The kelvin (K) is the SI base unit of thermodynamic temperature. It is one of the seven base units of the International System of Units, and it is the scale physics, chemistry, astronomy and engineering reach for when temperature has to mean something absolute.
Absolute zero is the starting point
The Kelvin scale starts at absolute zero, the lowest temperature possible, where the thermal motion of particles reaches its quantum-mechanical minimum. In kelvin that point is simply 0 K. In the scales most people grew up with it is −273.15 °C or −459.67 °F.
A kelvin is exactly the same size as a Celsius degree, so the two scales are just offset by 273.15: water freezes at 273.15 K and boils at 373.15 K (at standard atmospheric pressure). Unlike Celsius and Fahrenheit, which dip below zero every winter, a kelvin value is never negative. The scale has nowhere to go below its floor.
The scale is named after William Thomson, 1st Baron Kelvin (1824–1907), the Scottish-Irish physicist who proposed an absolute temperature scale in 1848 and did foundational work in thermodynamics. His title, in turn, comes from the River Kelvin, which runs past the University of Glasgow.
Why there are no "degrees Kelvin"
Until 1967 the unit was written °K and spoken as "degrees Kelvin". The 13th General Conference on Weights and Measures dropped the degree sign and the word: the unit is now the kelvin, lowercase, symbol K, used like any other SI unit. You write 300 K and say "three hundred kelvins" (or "kelvin"). The scale itself and the man keep their capital letter, so "the Kelvin scale" is correct while "300 degrees Kelvin" is not.
How the kelvin is defined today
For most of the twentieth century the kelvin was defined as 1⁄273.16 of the thermodynamic temperature of the triple point of water, the single combination of pressure and temperature at which ice, liquid water and vapour coexist. That made the unit depend on a specific sample of water with a specific isotope mix.
On 20 May 2019 the SI was redefined so that every base unit rests on a fixed constant of nature. The kelvin is now defined by fixing the Boltzmann constant at exactly 1.380649 × 10−23 joules per kelvin, which links temperature directly to the average kinetic energy of particles. The triple point of water is still 273.16 K to within experimental uncertainty, so nothing changed in everyday use, but the definition no longer depends on any physical artefact or substance.
Where kelvin temperature is used
- Physics and chemistry. The ideal gas law (
PV = nRT), the Boltzmann distribution, black-body radiation and most of thermodynamics only work with absolute temperature. Plug in Celsius and the equations give nonsense. - Astronomy. Stellar temperatures are quoted in kelvin: the Sun's surface is about 5,772 K, a cool red dwarf around 3,000 K, a blue giant tens of thousands. The cosmic microwave background, the afterglow of the Big Bang, is 2.725 K.
- Cryogenics. Liquid nitrogen boils at 77 K, liquid helium at 4.2 K, and superconducting magnets in MRI machines and particle accelerators live in that range. Nobody wants to do that arithmetic in negative Celsius.
- Lighting and photography. Colour temperature is a kelvin figure: a candle is about 1,850 K, a warm-white bulb 2,700 K, daylight 5,500 to 6,500 K. It describes the colour of the light a black body would emit at that temperature.
- Engineering and electronics. Thermal noise, semiconductor behaviour and heat-transfer calculations all use kelvin. Rankine, the absolute scale with Fahrenheit-sized degrees, survives in some US engineering fields for the same reason.
- Standards and calibration. Temperature standards, reference materials and the International Temperature Scale (ITS-90) are specified in kelvin, giving every laboratory the same absolute reference.
Kelvin at a glance
| Symbol | K (no degree sign) |
|---|---|
| Zero point | Absolute zero: 0 K = −273.15 °C = −459.67 °F |
| Size of one kelvin | Equal to one Celsius degree, 1.8 Fahrenheit degrees |
| Water freezes / boils | 273.15 K / 373.15 K |
| Named after | William Thomson, Lord Kelvin (proposed 1848) |
| Renamed from °K to K | 1967, 13th CGPM |
| Current definition | Boltzmann constant fixed at 1.380649 × 10−23 J/K (since 20 May 2019) |
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Kelvin Temperature is a small, free tool built by Kelvin Betances, an SEO specialist and hobbyist programmer who has spent his whole life being asked whether he is named after the temperature scale. (He is not. It is a coincidence he has decided to lean into.) The converter runs entirely in your browser, nothing you type is sent anywhere, and the source is on GitHub.