Calculate annual harvestable rainwater from your roof area and rainfall. Free online rainwater collection calculator with formula, examples, and FAQ.
The rainwater harvest calculator estimates how much water you can collect from your roof annually based on roof area, local rainfall, and collection system efficiency. Rainwater harvesting reduces municipal water usage, lowers bills, and provides a sustainable water source for gardens, lawns, and non-potable uses.
Annual Gallons = Roof Area (sq ft) × Annual Rainfall (in) × 0.623 × Efficiency
0.623 = gallons per inch of rain per sq ftOne inch of rainfall on one square foot of surface yields 0.623 gallons of water. This is derived from converting cubic inches to gallons (1 gallon = 231 cubic inches; 1 sq ft × 1 inch = 144 cubic inches; 144/231 = 0.623). Collection efficiency (typically 80-90%) accounts for losses from evaporation, splashing, and gutter overflow.
| Input | Output |
|---|---|
| 1500 sq ft roof, 40 inches rain, 85% efficiency | ~31,831 gallons/year (~2,653 gallons/month) |
| 2000 sq ft roof, 30 inches rain, 90% efficiency | ~33,642 gallons/year (~2,804 gallons/month) |
| 800 sq ft roof, 50 inches rain, 80% efficiency | ~19,936 gallons/year (~1,661 gallons/month) |