We expanded an existing model for permeability in mudrocks and shaly sands to include computation of effective grain radius and the Archie's law parameter in granular media composed of two different grain sizes. We found that the effective grain radius is the harmonic mean of the endmember grain radii and that can be computed as the geometric mean of the endmember values. We tested our model with three-dimensional lattice-Boltzmann simulations of flow through dilute and concentrated systems, and with comparison to measurements of laboratory-prepared and natural samples as well as field data. Modeled permeabilities matched the simulated and measured permeabilities over a wide range of porosities, grain sizes, and grain shapes. We additionally found that the model is independent of grain packing and aspect ratio, as these parameters only affect the endmember values. Our predicted permeabilities generally fall within previously determined bounds, and we derive approximations for permeability as a function of endmember permeability for cases when endmember values are equal and when endmember grain radii are very different. Our results advance our understanding of permeability in heterogeneous porous media.