This paper explores optimal tank size for domestic rainwater harvesting systems in Asian tropical climates. A total of 128 locations in Vietnam covering three regional climate patterns were selected for the study. The system behavior was simulated on a daily basis using between 27 and 32 years of rainfall data. Annual water cost was investigated to determine optimal tank size. The relationship among optimal size, climate and system conditions was also analyzed. Results of the study emphasize the economic benefit of rainwater harvesting for the whole study area. The optimal tank size for a non-potable rainwater harvesting system has a range of 1.2–2.6 m3, exhibiting 19–65% supply efficiency and a payback period of 7–17 years. Extended system scenarios reveal a contrast in the influences of demand and roof area on optimal size in relation to rainfall amount. The roof area is critical in determining optimal size in the low rainfall area while the demand is important in the high rainfall area. Although there is a certain degree of variability in optimal tank size, it does not considerably undermine the economic benefit of a rainwater harvesting system.