A volumetric constitutive law for snow is developed by considering the deformation of the ice grains and grain bonds which form the porous material. The equations of equilibrium and mass conservation are applied to both the grain body and neck regions to calculate the rate of change of grain geometry and neck geometry. The matrix material, ice, is assumed to be a nonlinear viscoplastic material. Comparison with data shows excellent agreement for a wide range of initial densities and for large volumetric deformations. Calculations are also made to evaluate grain and neck deformation during compaction. The model can be applied to porous metals and foams, although the constitutive law for the matrix material would have to be altered.