On the left is a glass thermometer. The column rises from absolute zero to 373 K and falls back; near zero a blue mark flashes. On the right, gas molecules move in a vessel: blue ones slowly, orange ones fast. The letter K shows through the vessel. Below, the display reads 273.16 K — the triple point of water. A recording light blinks.
Quantity data sheet
SI base unitrev. 2026-10Kelvin K
SI base unit · quantity: thermodynamic temperature — a measure of the average energy of particle motion
Why nothing is colder than −273.15 °C
In the 18th century physicists noticed that when gas in a closed vessel is cooled, its pressure drops by the same fraction with every degree. Extending this straight line on a graph, they found the point where the pressure would become zero — about −273 °C. Nothing can be colder: the molecules have no further to slow down.
The Kelvin scale starts at this point — absolute zero. Its divisions are the same as on the Celsius scale, so 0 °C is 273.15 K, and water boils at normal pressure at about 373.12 K. Negative kelvins do not exist in the ordinary sense.
Since 2019 the kelvin has been defined through the Boltzmann constant k = 1.380 649·10⁻²³ J/K. It links temperature to energy. At temperature T each gas molecule carries on average a kinetic energy of 3/2·k·T. A thermometer essentially measures exactly that.
The symbol of the kelvin is a capital K without a degree sign: “300 K”, not “300 °K”. The quantity is written with a capital T. The name of the unit is lowercase — kelvin — although it is named after Lord Kelvin. A lowercase k is the Boltzmann constant and the prefix kilo.
The kelvin and the degree Celsius are the same size but have different zeros. 20 °C is 293.15 K. A temperature difference is the same in both units: warmer by 5 °C means warmer by 5 K. Do not confuse temperature with heat either: temperature is not energy, but a measure of its average share per particle.
01 · Definition
particle energy · gas · glowMolecules fly about in a gas, jostle in a liquid and tremble in place in a solid. The faster they move, the higher the temperature. Temperature in kelvins is directly proportional to the average kinetic energy of this motion: twice the kelvins, twice the energy.
That is why a gas’s pressure and volume grow in direct proportion to temperature in kelvins, not in degrees Celsius. Warming the air in a tyre from 0 to 27 °C is not “infinitely more” but from 273 to 300 K: the pressure rose by about 10 %.
A hot body glows. The higher the temperature, the shorter the wavelength that carries most of the light. Embers glow dark red at 1000 K, a lamp filament yellow-white at 2700 K, the surface of the Sun white at 5772 K. Temperature can be measured by the colour of the glow, without touching the body.
What changes as the kelvins rise
Use the slider to set the temperature from 1 to 6000 K. Gas molecules move in the vessel on the left: their average speed grows as the square root of temperature, and the pressure on the walls grows in direct proportion to it. A molecule’s colour shows its speed: blue is slow, orange is fast. On the right is a tungsten sphere: as the temperature rises it begins to glow, first dark red, then yellow and white. Choose the gas — helium, nitrogen or xenon: at the same temperature, light molecules move faster. Below are three views. “Pressure” is the straight line of p against T and the point of absolute zero. “Speeds” is the distribution of molecules by speed. “Glow” is the spectrum of a hot body and its peak.
Every step towards zero is harder than the last
To cool a body, you have to take heat from it with something even colder. The closer to zero, the less heat is left and the harder it is to remove. Lasers and magnetic traps cool clouds of atoms to billionths of a kelvin, but zero itself remains a limit that can only be approached.
02 · Conversion
scales · energy · reference pointsFrom kelvins to degrees and to energy
Enter a temperature on any scale. The first tab converts it into kelvins and degrees Celsius, Fahrenheit and Rankine. The second shows the energy of a particle and the colour of the glow. The third shows the reference points of the ITS-90 scale.
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Temperatures in the Universe differ by a factor of almost 10¹⁷. The coldest clouds of atoms in laboratories have been cooled to hundreds of picokelvins; the core of the Sun is hotter than 15 million kelvins. People live in a narrow band around 300 K, where water can be liquid.
04 · Measuring instruments
cell · gas thermometer · pyrometer · acousticTemperature is checked against points where a substance changes
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05 · Writing rules
K without a degree sign · °C with oneK is written without a degree sign
The kelvin is written as a capital K, with a space after the number and without “°”. The degree Celsius is written “°C”, also with a space. A temperature difference can be written in kelvins or in degrees Celsius — the numbers are the same.
Since 1967 the kelvin is not a degree, and no “°” is placed before K. Without “°”, a capital C is the coulomb. The prefix and symbol are written in Latin letters with the right case: mK, not mk. The Boltzmann constant has the unit J/K. The name of the unit is lowercase, and “degree Kelvin” is an obsolete name.
06 · Neighbouring units
thermodynamicsThe same step, zero shifted by 273.15.
Jk·T — energy per particle.
molR = N_A·k links gas and temperature.
07 · Historical section
archive · from Charles’s gas to the Boltzmann constantHow the zero that cannot be reached was found
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The water point became measured
From 1954 to 2019 the kelvin was defined like this: the triple point of water is exactly 273.16 K. At this point ice, liquid water and vapour exist together. It is reproduced in a glass cell with very pure water, and each country had its own set of such cells.
But water turned out to be a fussy standard: the temperature of the triple point depends on the isotopes of hydrogen and oxygen in it. So in 2019 the kelvin was detached from water and defined through the Boltzmann constant. Now 273.16 K is a measured number, with an uncertainty of about 0.1 mK.
Nothing changed for ordinary thermometers. On the right are two steps of one scale: its anchor used to be water, now it is the energy of a particle, and water has become one of the reference points.
Catalogue of quantities
Temperature and heat · 12 unit sheets
Index · all quantities on one pageThe lab at the entrance to the indexA drop of acid changes the colour of the juice, salt colours the flame, water freezes at a tap. Below them are all Simetrium sheets, by discipline and alphabetically.Open the index →
