An equation is presented for the prediction of the Poisson’s ratio of porous materials. The equation is strictly derived for spherical porosity and isotropic materials and it is valid for the whole porosity range. for low porosity, the equation coincides with a published equation, which has been verified in the past by comparison with extensive experimental data. For the high-porosity range, the theoretical variation of the Poisson’s ratio exhibits a trend converging to a value nu(P)=0.5, when the porosity increases to P=1. A similar converging trend has been found in other theoretical studies, but a rigorous experimental verification of such variations has still to be carried out.