A general phenomenological formulation for drop size distribution (DSD), written down as a scaling law, is proposed. It accounts for all previous fitted DSDs. As a main implication of the expression proposed, the integral rainfall variables are related by power functions and agree with experimental evidence. Additional consequences are also analyzed. From this formulation there follows a general methodology for scaling all data in a unique plot, leading to more robust fits of the DSD. An illustrative example on real data is provided.