PACS 81.10.Aj -Theory and models of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation PACS 64.30.Ef -Equations of state of pure metals and alloys Abstract -A grain topology-size equation for three-dimensional grains is suggested by considering the interactions of the nearest-neighbor grains, which indicates that the average grain size of a given topological class is determined by the difference between the grain face number and the average face number of nearest neighbors. In addition, a practical grain topology-size equation is presented. The two equations show good agreement with various experimental results. They are also verified by data of large-scale Potts model Monte Carlo simulation and Surface Evolver simulation.