ChemixaLEARN CHEMISTRY

Kelvin (K)

Quantity: thermodynamic temperature · Category: SI base unit

The kelvin is the SI base unit of thermodynamic temperature. Since 2019 it has been defined by fixing the numerical value of the Boltzmann constant at exactly 1.380649×10⁻²³ J/K; absolute zero (0 K) is the lowest possible temperature. The kelvin is never written with a degree sign and never called 'degrees Kelvin' — write '273.15 K', not '273.15 °K'.

Relation to SI

0 °C = 273.15 K; Δ1 K = Δ1 °C

Examples

  • Room temperature 25 °C = 298.15 K
  • Gas-law calculations always use kelvin: PV = nRT with T in K
  • Water freezes at 273.15 K and boils at 373.15 K (at 1 atm)