The kelvin, symbol \(\pu{K}\), is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant \(k\) to be \(\pu{1.380649E-23}\) when expressed in the unit \(\pu{J K-1}\), which is equal to \(\pu{kg m2 s-2 K-1}\), where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.