Onsite rainwater harvesting is an inexpensive water source for indoor use; it is also a sustainable water management practice, since it may contribute to the reduction of runoff volume and peaks, and to the control of non-point source pollution. The aim of this paper is to provide a method for sizing rainwater harvesting tanks based on the local daily rainfall record, the served family size, the roof collection area, the roof type, the in-house water uses, and other factors. Various methods used worldwide are presented and compared. The comparative application of these methods is made using a daily rainfall record from a meteorological station near Kimmeria Village in Xanthi, Northeast Greece. Based on the proposed method here, the optimum tank size is computed by allowing excess water to overflow and setting to zero the use of the public water supply; the method may result in reduced tank sizes compared to other methods tested.