Molar refractivity,^[1] ${\displaystyle A}$, is a measure of the total polarizability of a mole of a substance and is dependent on the temperature, the index of refraction, and the pressure.

The molar refractivity is defined as

${\displaystyle A={\frac {4\pi }{3))N_{A}\alpha ,}$

where ${\displaystyle N_{A}\approx 6.022\times 10^{23))$ is the Avogadro constant and ${\displaystyle \alpha }$ is the mean polarizability of a molecule.

Substituting the molar refractivity into the Lorentz-Lorenz formula gives, for gasses

${\displaystyle A={\frac {RT}{p)){\frac {n^{2}-1}{n^{2}+2))}$

where ${\displaystyle n}$ is the refractive index, ${\displaystyle p}$ is the pressure of the gas, ${\displaystyle R}$ is the universal gas constant, and ${\displaystyle T}$ is the (absolute) temperature. For a gas, ${\displaystyle n^{2}\approx 1}$, so the molar refractivity can be approximated by

${\displaystyle A={\frac {RT}{p)){\frac {n^{2}-1}{3)).}$

In SI units, ${\displaystyle R}$ has units of J mol⁻¹ K⁻¹, ${\displaystyle T}$ has units K, ${\displaystyle n}$ has no units, and ${\displaystyle p}$ has units of Pa, so the units of ${\displaystyle A}$ are m³ mol⁻¹.

In terms of density ρ, molecular weight M, it can be shown that:

${\displaystyle A={\frac {M}{\rho )){\frac {n^{2}-1}{n^{2}+2))\approx {\frac {M}{\rho )){\frac {n^{2}-1}{3)).}$