Brazilian semi-arid productive techniques use rainwater harvesting systems, which are a sustainable water management practice with a low environmental impact and have been adopted as an alternative to meet water demand worldwide. The aim of this study was to find an optimal sizing methodology for rainwater harvesting systems using local parameters allied to the lowest system cost. The analyses were based on a system supplying 95% of a 250 L/d demand for a goat herd in Feira de Santana, Bahia, Brazil. The area available for system implementation did not limit the analysis. The only limiting parameters were the quantity and quality of water required for the herd. The results indicated that, among the combinations of catchment area and water tank volume capable of meeting the defined demand, there is an optimal set, with minimum cost. This was a catchment covering 220 m² with a 21.1 m³ water tank, equivalent to a 0.62 demand fraction (FD). The variance influence of meeting service efficiency and demand, in the system's implementation and performance, was also analyzed.