We show that the absolute determination of the refractive index when combined with an expansion technique for obtaining the higher‐order coefficients of the Lorentz–Lorenz expansion leads to precise values of density. The compressibility factor of methane calculated with the first three coefficients of the Lorentz–Lorenz expansion is comparable to the best PVT values. The optical method for the determination of the compressibility factor is shown to be not only precise, but also has the ability to produce numerous experimental points in a short time as compared to other methods.