<p>Rainwater harvesting (RWH) for indoor use could be a useful practice for a sustainable management of urban water. The impact of the temporal patterns of rainfall on the size of the tank of an RWH system has been analysed in the present work. This analysis was carried out through numerical simulations and was focused on the national Italian territory. Daily rainfall data from 3436 rainfall gauge stations located throughout the Italian territory were considered, and buildings with different catchment areas and numbers of users were considered. The coefficient of variation of the daily rainfall data was used to quantify the temporal variability of the rainfall. Furthermore, analytical expressions have been developed in this work to compute the performance of a rainwater harvesting system (water savings, retention efficiency and overflows) as a function of certain characteristics of the building and of the rainfall regime. The results of the numerical simulations point out the important role of the temporal patterns of rainfall in the design steps of such a system and successfully support the analytical expressions proposed for the performance indicators.</p>