Rainwater Harvesting Calculator
Estimate annual rainwater collection potential based on roof area and local rainfall.
sq ft
inches
%
USD/1000 gal
Annual Rainwater Collected
29,904 gallons
Monthly Average2,492 gallons
Annual Water Savings150 USD
Recommended Tank Size2,500 gallons
Annual Rainwater Collected vs Collection Roof Area
How to Calculate Rainwater Collection
Rainwater harvesting captures roof runoff for irrigation, toilet flushing, or other non-potable uses.
Formula
Gallons = Roof Area (sq ft) * Rainfall (inches) * 0.623 * Collection Efficiency
The factor 0.623 converts 1 inch of rain on 1 sq ft to gallons. Efficiency accounts for gutter losses, first-flush diversion, and evaporation.
Example Calculation
1,500 sq ft roof in an area with 40 inches annual rainfall, 80% collection efficiency, $5/1000 gal water cost.
- 01Annual collection: 1,500 * 40 * 0.623 * 0.80 = 29,904 gallons
- 02Monthly average: 29,904 / 12 = 2,492 gallons
- 03Annual savings: 29,904 * $5 / 1,000 = $150
- 04Recommended tank: ceil(2,492 / 100) * 100 = 2,500 gallons