In this paper we introduce an effective medium theory that is capable of describing site percolation in both lattice and continuum three-dimensional systems. By exploiting self-consistency with a complete graph or network of identical conductors, the resulting effective medium equation allows for a straightforward estimate of the percolation threshold, which compares well with the available numerical results. The present formalism is easily generalized to describe transport in conductor-insulator composites with different microstructure properties. Copyright (C) EPLA, 2011