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Abstract. Rainwater harvesting systems (RWHS) are used in water supply for reasons which vary from supplemental irrigation to emergency relief. Key to successful design of RWHS is incorporation of the stakeholder‘s goals. Often it is desirable to present a decision-making tool to a stakeholder so they can compare possibilities and choose the optimal solution for themselves. Such a tool is most accessible and useful when presented in the same units with which the stakeholder is concerned. We developed a decision-making tool defining water use rate, water supply reliability, and system cost relationships bounded by the system‘s physical constraints of precipitation and accessible roof area. This process is illustrated with a case study of Guaynabo, Puerto Rico. The decision-making tool is given in the form of figures which would allow a stakeholder to graphically see what parameter combinations are possible and select their preferred RWHS size accordingly. Water use rates are given in units of number of users and reliability is color coded. We determined that the case study RWHS could supply one toilet flush/day for ≤ 200 people with 85+% reliability when sized at 5,000 gallons, but reliability decreased for higher demands and larger systems serving more users. During non-emergency operation the RWHS could supplement up to 85% of irrigation needs not met by direct rainfall, providing water and cost savings. More broadly, the process can be used to design and construct systems that support Food, Energy, and Water needs to bolster disaster relief and resilience efforts at the community level.