The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 x 10^–23 when expressed in the unit J K^–1, which is equal to kg m² s^–2 K^–1, where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.