Abstract Simple formulae are given for the surface area, volume, center of mass, moments of inertia, and principal axes of any homogeneous polyhedron. These can easily be combined with algorithms for the gravitational field of such polyhedra. A FORTRAN version of our code (available from the author) is used to compare digital models of Phobos at different resolutions. The results indicate that a surface resolution of about 10° is adequate to characterize the inertial properties of a small satellite or asteroid within a few percent, comparable to the systematic uncertainty for Phobos. This accuracy scales as the square of the resolution.