This Excel-based calculator was developed to help residents of Washington State size and test scenarios for rain water harvesting systems based on the typical climate. The calculator assumes that water is harvested entirely from roofs and that 80% of precipitation falling on roofs is captured. Inputs are the nearest Washington city, estimated daily indoor water use, area of lawn to be irrigated, cistern storage capacity, and roof area. Ouputs include monthly cistern carryover and overflow volumes and estimated monthly volumes of captured rainwater for indoor and outdoor use. Results are displayed in tabular and bar graph format.
https://ecology.wa.gov/Water-Shorelines/Water-supply/Water-recovery-solutions/Rainwater-collection (scroll down for a link to the Excel file)