In statistical mechanics, Boltzmann's equation is a probability equation relating the entropy S S, also written as S B {\displaystyle S_{\mathrm {B} }}, of an ideal gas to the multiplicity, the number of real microstates corresponding to the gas's macrostate: where k B {\displaystyle k_{\mathrm {B} }} is the Boltzmann constant and equal to 1.380649 × 10−23 J/K, and ln \ln is the natural logarithm function. Wikipedia