A semi-theoretical study of porosity of particulate beds composed of mixtures of various size spherical particles has been performed. A model for mixtures of only two particle sizes is developed and is used to identify several dimensionless parameters which are correlated by comparing predictions with the data. The model is then extended to evaluate porosity of mixtures of a large number of particle sizes and distributions. The predictions from the model are found to compare quite well with the data taken in this work and the existing data in the literature when unavailability of a certain fraction of pores formed of largest size particles in the mixture is accounted for.