This paper presents an investment feasibility analysis of rainwater harvesting systems for the residential sector in Santa Catarina State, southern Brazil. Five towns were selected for the analysis. Daily rainfall data, average water consumption, rainwater tank costs, water pump costs, and water, sewage and electricity tariffs were obtained for the five towns. Different values for the roof area, number of residents, potable water demand, and rainwater demand were analysed. The rainwater tank capacities were estimated using the Netuno computer programme. The investment feasibility analysis was performed considering the ideal lower tank capacity, as well as capacities smaller and larger than the ideal. Such capacities were obtained according to a variation of six months in the payback period. It was observed that the ideal tank capacity can be conservative for high rainwater demands. In such cases, an investment feasibility analysis should be performed in order to obtain a more appropriate tank capacity. The main conclusion is that rainwater usage is economically feasible for most cases; and the higher the rainwater demand, the higher the feasibility.