Boltzmann's entropy formula

In statistical mechanics, Boltzmann's equation (also known as the Boltzmann–Planck equation) is a probability equation relating the entropy ${\displaystyle S}$, also written as ${\displaystyle S_{\mathrm {B} }}$, of an ideal gas to the multiplicity (commonly denoted as ${\displaystyle \Omega }$ or ${\displaystyle W}$), the number of real microstates corresponding to the gas's macrostate:

${\displaystyle S=k_{\mathrm {B} }\ln W}$

(1)

where ${\displaystyle k_{\mathrm {B} }}$ is the Boltzmann constant (also written as simply ${\displaystyle k}$) and equal to 1.380649 × 10⁻²³ J/K, and ${\displaystyle \ln }$ is the natural logarithm function (or log base e, as in the image above).

In short, the Boltzmann formula shows the relationship between entropy and the number of ways the atoms or molecules of a certain kind of thermodynamic system can be arranged.