How Rainwater Harvesting is Calculated
Rainwater harvesting is a key sustainable water management technique. In drought-prone regions and suburban homesteads alike, it provides an abundant, cost-effective water supply for garden irrigation, car washing, and domestic non-potable needs.
The fundamental calculation is simple: Harvested Volume (Liters) = Precipitation (mm) × Catchment Area (m²) × Runoff Efficiency (or Gallons = Precipitation (in) × Area (sq ft) × 0.623 × Efficiency). Smooth metal or glazed tile roofs yield up to 95% runoff efficiency, while asphalt shingles average 85% and concrete surfaces yield roughly 70%.
Example: A 1,500 sq ft (approx. 140 m²) roof receiving 1 inch (25 mm) of rain at 85% efficiency harvests approximately 800 gallons (3,000 liters) of clean runoff—enough to supply a home garden for weeks.
Across the United States, average annual precipitation ranges from under 10 inches in desert regions to over 50 inches along the East Coast and Pacific Northwest. This tool allows homeowners, growers, and environmental planners to size rain barrels and cistern systems accurately.