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

Two rules circulate here and they give wildly different answers, so it is worth being clear about which question each one answers.

The buffer rule sizes the tank to ride out the gaps between rain events: hold 2 to 6 weeks of your average collection. That suits a temperate climate where rain arrives most months and the tank is topping up constantly.

The dry-season rule sizes the tank to carry water from the wet season into the dry one: hold enough to cover your demand through the longest dry spell. In a Mediterranean or monsoon climate that can mean 30 to 50% of the annual collection, which is often ten times the buffer figure.

Pick by climate, not by preference. A London roof needs the buffer rule. A Perth or Los Angeles roof needs the dry-season rule, because between May and October essentially nothing falls and a tank sized for a fortnight is empty by June.

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

What it saves

Rainwater displaces metered water, so the saving is simply the volume you actually use times your water rate. The catch is that you only save on water you would otherwise have bought: a tank that overflows in March has collected nothing of value if your garden is dormant.

Metered rates vary enormously, from under $1 per 1,000 L in parts of the US Midwest to over $5 in Denmark or parts of Australia, and many bills charge sewerage on top by assuming everything metered in goes back out. Check your own bill for the combined figure rather than the headline water rate, because a rainwater system that feeds a garden avoids both charges.

Payback on a simple barrel-and-downpipe setup is usually a season or two. Payback on a 20,000 L tank with pumps, filtration and buried pipework is often measured in decades, and people who install those are generally buying independence rather than savings.