Kelvin

The kelvin (symbol K) is the SI unit of thermodynamic temperature. Zero kelvin is absolute zero, the lowest possible temperature, and one kelvin is the same size as one degree Celsius.

The kelvin scale starts at absolute zero, so there are no negative values, and water freezes at 273.15 K and boils at about 373 K at standard pressure. To convert from Celsius, add 273.15. The unit is named for William Thomson, Lord Kelvin, who proposed an absolute temperature scale in 1848 based on the laws of thermodynamics.

Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin. This links temperature directly to energy. Earlier it was defined by the triple point of water, 273.16 K, a temperature at which ice, liquid and vapor coexist.

The cosmic microwave background is about 2.725 K, liquid nitrogen boils near 77 K, a room is near 293 K, and the visible surface of the Sun is about 5,770 K. Kelvin is also used for the color temperature of light, and in physics it appears in laws such as the specific heat capacity relations and entropy.

• Boltzmann Constant — the constant that now defines the kelvin
• Thermodynamics — the science that gave rise to the scale
• Entropy — a quantity measured relative to temperature