Rainwater harvesting gains more and more ground as a modern, relatively inexpensive and simple water-saving technology, and as a sustainable water management practice, which saves water, and reduces stormwater runoff and peaks and non-point source pollution. In this paper, in order to determine the optimal size of rainwater harvesting tanks, two methods, the daily water balance method and the dry period demand method, are used in 75 regions of Greece to meet 30, 40 and 50 % of total water demands of households of 3 to 5 residents. The daily water balance method was developed based on a heuristic algorithm which uses the daily rainfall data, the rainfall collection area, the runoff coefficient, the available storage volume and the water demands, allowing excess water to overflow and setting public water supply to zero. The dry period demand method is based on meeting demand for the longest annual average dry period. According to the daily water balance method, in the majority of the 75 regions studied, tank sizes up to 50 m(3) can meet a 240 L/day demand (40 % of total daily demand of 4 residents) with roof area not exceeding 300 m(2). More than 50 m(3) tank size is needed to meet demands of 300 L/day (40 % of 5 or 50 % of 4 residents) or 375 L/day (50 % of 5 residents). Results demonstrate that the tank size is strongly affected by the dry period length; small dry periods lead to small tanks, with the exception of low rainfall-high demand (300-375 L/day) case, where low rainfall increases sizes, having the dominant role. Comparison among the dry period demand and the daily water balance methods showed that in all cases, the dry period demand method calculates smaller tanks, with the exception of areas with medium-high rainfall and high dry period or low-medium demand (135-225 L/day) and high roof areas (more than 300 m(2)). Therefore, the main conclusion is that the rainwater harvesting tank capacity is strongly affected by various local variables and cannot be formulated. However, the method presented here can be programmed in a spreadsheet with no much effort, making harvesting tank computations easy.