Methane diffusion in micro- and mesopores of carbonaceous materials is dominated by molecular interactions with the pore walls. As a consequence, the fluid molecules are mainly in a diffusive regime and the laws of fluid mechanics are not directly applicable. A method called the “free volume theory” has been successfully used by different authors to study the diffusion of n-alkanes into microporous carbons. However, we show in this paper that such a method fails to describe the dynamical properties of methane in porous hosts presenting both micro- and mesopores. We further evidence that this theory is limited to structures whose pore diameters are lower than ∼3 nm. We then propose a simple scaling method based on the micro- and mesoporous volume fraction in order to predict diffusion coefficients. This method only requires the knowledge of (i) the host microporous volume fraction and (ii) the self-diffusion coefficient in micropores smaller than 3 nm, which can be obtained using the “free volume theory”, quasi-elastic neutron scattering experiments, or atomistic simulations.