Combining a polynomial free-form surface representation with Gauss' divergence theorem allows efficient and exact calculation of the moments of the enclosed objects. For example, for an cubic representation, volume, center of mass, and the inertia tensor can be computed in seconds even for complex objects with serval thousand patches while change due to local modification of the surface geometry can be computed in real-time as feedback for animation or design. Speed and simplicity of the approach allow solving the inverse problem of modeling to match prescribed moments.