Rainwater Harvesting Calculator

Rainwater Collection Formula Volume collected = Roof area × Rainfall × Runoff coefficient Where runoff coefficient accounts for evaporation, first-flush loss, and roof imperfections.

Runoff Coefficients by Roof Type Metal (steel, aluminum): 0.90 — very efficient Tile (clay, concrete): 0.85 — good Asphalt shingles: 0.80 — common residential Gravel-covered flat roof: 0.70 — moderate Green roof: 0.40–0.60 — much lower due to absorption Treated wood shingles: 0.75

First Flush Diverter The first 1–2 mm of rainfall washes dust, bird droppings, and contaminants off the roof. First-flush volume to discard: Roof area (m²) × 0.001 m = liters After first-flush, water quality improves significantly. For drinking water: additional filtration (sediment filter, UV sterilizer) required.

Typical Annual Rainfall by Region Arid/desert (Phoenix, AZ): 200 mm/yr | Semi-arid (Denver, CO): 400 mm/yr Mediterranean (Los Angeles): 380 mm/yr | Temperate (London, UK): 600 mm/yr Subtropical (Sydney, AU): 1,200 mm/yr | Tropical (Miami, FL): 1,500 mm/yr Wet tropical (Manila): 2,400 mm/yr | Rainforest (Hilo, HI): 3,800 mm/yr

Tank Sizing Tank should store at least 2–4 weeks of your average collection rate. Oversize for dry seasons: aim to hold 30–50% of annual collection if possible. Common sizes: 200 L barrel, 1,000 L IBC tote, 5,000–20,000 L poly tanks.

Uses for Collected Rainwater Garden irrigation (most common), toilet flushing, laundry (with filter), car washing. With proper treatment: drinking water (check local regulations). 1 person garden: 5–10 L/day | Full household non-potable: 50–100 L/day