This rainwater harvesting calculator estimates how much rainwater can be collected from a roof per rainfall event.
Formula to calculate gallons of rainwater harvested:
Gallons = Roof Area (sq ft) x Rainfall (inches) x 0.623 x Runoff Efficiency
Where:
- 0.623 = gallons per square foot per inch of rain
- Runoff efficiency is usually ~0.85 (or 85% – enter into this calculator as percent, not decimal)