Molar refractivity, , 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
where  is the Avogadro constant and  is the mean polarizability of a molecule.
Substituting the molar refractivity into the Lorentz-Lorenz formula gives, for gasses
where  is the refractive index,  is the pressure of the gas,  is the universal gas constant, and  is the (absolute) temperature. For a gas, , so the molar refractivity can be approximated by
In SI units,  has units of J mol K,  has units K,  has no units, and  has units of Pa, so the units of  are m mol.
In terms of density ρ, molecular weight M, it can be shown that: