The current study describes the development of a Decision Support System (DSS) for estimating the optimal tank size of rainwater harvesting systems (RHS). Site-specific conditions are incorporated into the analysis, including those of the local rainfall regime and domestic demand for water, in addition to the desired level of confidence. The developed DSS is accessible via the interface of a web application, in which the annual efficiency of potential water savings by RHSs is analyzed by a simulation based on historical rainfall data and rainfall distribution. The developed DSS was applied and validated for nine case studies in the State of Mexico, considering three distinct rainfall regimes and catchments area. According to estimates, an annual potential water savings of up to 46,500 L per household may be achieved under average conditions. (C) 2017 Elsevier Ltd. All rights reserved.