Kelvin
SI base unit · quantity: thermodynamic temperature
01 · Definition
The kelvin is the only unit that measures temperature from the true beginning of the scale rather than from some convenient zero. And the only one defined through energy: temperature in the SI is the energy of thermal motion, nothing more.
Since 2019 the kelvin has been set by fixing the Boltzmann constant: k = 1.380 649·10⁻²³ joule per kelvin. One kelvin is the change of temperature at which the thermal energy kT changes by exactly 1.380 649·10⁻²³ joule. Zero on this scale is not a convention but the point where thermal motion ceases.
There is no negative absolute temperature: at 0 K thermal motion stops. Reaching zero is impossible — the third law of thermodynamics. The laboratory record is 38 picokelvins, but not zero.
02 · Conversion
Enter a temperature — the passport will convert it
The kelvin is the only temperature scale for which SI prefixes make sense: millikelvins in a cryostat, megakelvins in a star. Celsius and Fahrenheit have a shifted zero, so they take neither prefixes nor multiplication — only a shift and a factor.
In practice temperature is not computed from the Boltzmann constant but checked against the fixed points of ITS-90: the triple point of water, the freezing points of tin, zinc, silver and gold — seventeen fixed values.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| µK | microkelvin | 2.95·10⁸ µK | laser cooling, atom traps |
| mK | millikelvin | 295 000 mK | cryostats, quantum computers |
| K | kelvin | 295 K | the base notation of temperature in SI |
| kK | kilokelvin | 0.295 kK | plasma, arcs, stellar surfaces |
| MK | megakelvin | 2.95·10⁻⁴ MK | stellar cores, fusion facilities |
SI prefixes work only with the kelvin: Celsius and Fahrenheit have a shifted zero, and prefixes are not applied to them
03 · Orders of magnitude
from picokelvins to quark–gluon plasmaRoom temperature, 295 K
04 · Measuring instruments
What kelvins are measured with
The speed of sound in a noble gas is directly tied to kT. The primary method by which the Boltzmann constant itself was measured before it was fixed.
Johnson thermal noise in a resistance is proportional to kT. It works from millikelvins to thousands of kelvins without calibration.
It reads temperature from the radiation spectrum via Planck's law — that is how furnaces, molten metal in casting and stellar surfaces are measured.
An ampoule of water where ice, water and vapour hold 273.16 K on their own. The working point for transferring the scale, reproducible to 0.1 mK.
05 · Writing rules
Just K — no degree sign and no full stop
The sign «°K» was abolished in 1967: the kelvin is not a degree but a unit in its own right, so one writes 300 K. The symbol is capital because it is a surname; the unit name is lowercase: «three hundred kelvins». Temperature differences in the SI are written in kelvins, even when the temperatures themselves are in Celsius.
06 · Neighbouring units
The kelvin stands next to the joule — the Boltzmann constant links them directly. The degree Celsius is the only derived unit whose definition contains the kelvin: it is the same size of step, only with a zero shifted to 273.15 K.
At room temperature kT ≈ 0.025 eV — this value sets the threshold for semiconductors and chemical reactions.
07 · Historical section
How absolute zero was searched for
William Thomson proposed a scale whose zero is the cessation of thermal motion, with the step taken from Celsius. It was written «°A» — absolute.
The same idea of absolute zero, but with the Fahrenheit step. It lives on in American heat engineering: 1 K = 1.8 °Ra.
The kelvin was 1/273.16 of the temperature of the triple point — that is, it depended on the properties of a single substance and its isotopic composition.
With the degree sign and the word «degree». Abolished in 1967: the kelvin is not a degree but a base unit with a zero of its own.
The kelvin got rid of water, but water stayed on the job
The definition via the triple point of water had an awkward detail: the temperature of that point depends on isotopic composition — heavy water freezes differently from ordinary water, and a reference composition of ocean water had to be specified. With the Boltzmann constant fixed, the definition became free of any substance, yet in practice the triple-point cell is still the workhorse: the primary realisation is expensive, and the ampoule of water reproduces 273.16 K to a tenth of a millikelvin.
Catalogue · The International System of Units
A passport for every quantity
Seven base units, twenty-two derived units with names of their own, and an archive of systems withdrawn from use. Each is a sheet of its own: definition, dimension, conversion, instruments, writing rules. In 36 languages.
Base units
open the kelvin's passportDerived units with names of their own
in terracotta — the only one defined through the kelvinHistorical systems
archive of notationsPassport language
36 languages. The symbol K and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.