Streets and roofs of buildings in cities will act as an obstacle to the penetration of rainwater into the soil, resulting in increased runoff in the city. One of the practical solutions that can be effective in addition to meet part of the non-potable needs of the residents is to reduce the runoff of the city's surface and its damages, as well as the Performance of rainwater harvesting systems. In this method, a large part of runoff is stored in the tanks. In this paper, by simulating the roofing surfaces system of residential buildings in Matlab, optimization of rainwater storage tanks in Karaj and Ahvaz was investigated. According to optimal tank volume, rainfall statistics and specifications of residential buildings, to analyze the volume of water can be saved and volume of overflowed water were evaluated and an estimate of the number of days of the year that can be used by this runoff to meet the non-potable needs. The results showed that although average annual rainfall of Ahvaz is lower than Karaj, the overflow ratio in Ahvaz is higher due to the irregularity of the precipitation process in this city and the percentage of precipitation to runoff is higher.