The objective of this study is to assess the investment feasibility analysis of rainwater harvesting in a building located in Brazil. Rainwater was used to supply toilets, cleaning and irrigation. The costs of materials, labour and maintenance were obtained to carry out the economic analysis; the indicators used were the net present value, internal rate of return and payback period. The investment feasibility analysis and the potential for potable water savings were obtained by means of computer simulation. The potential for potable water savings ranged from 26.70% to 64.70%. The net present value, internal rate of return and payback period for the best scenario were, respectively, R$132,801.47, 3.73% per month and 32 months. For the worst scenario the net present value was R$9,451.26, the internal rate of return was 0.91% per month and the payback period was 166 months. Thus, rainwater can be used as a sustainable alternative and be financially feasible.