Rainwater Harvesting Calculator
Calculate how many gallons of rainwater your roof can collect each year based on area, local rainfall, and system efficiency. Use it to size a storage tank or estimate irrigation potential.
About this calculator
Rainwater harvesting captures precipitation from a roof surface and stores it for irrigation, toilet flushing, or other non-potable uses. The volume collected is governed by the formula: Gallons = roof_area × annual_rainfall × 0.623 × (collection_efficiency / 100). The conversion factor 0.623 converts the product of square feet and inches of rainfall into US gallons (1 inch of rain on 1 sq ft = 0.623 gallons). Collection efficiency accounts for losses from evaporation, first-flush diverters, and roof surface absorption, and is expressed as a percentage. A metal roof in a clean environment may achieve 90% efficiency, while a gravel-covered roof might only reach 70%. Understanding each variable helps you right-size your storage tank and set realistic expectations for how much water you can offset.
How to use
Assume a roof area of 1,500 sq ft, annual rainfall of 30 inches, and a collection efficiency of 80%. Step 1 — base volume: 1,500 × 30 = 45,000 sq ft·inches. Step 2 — convert to gallons: 45,000 × 0.623 = 28,035 gallons. Step 3 — apply efficiency: 28,035 × (80/100) = 22,428 gallons per year. That is roughly 1,869 gallons per month — enough to handle significant landscape irrigation needs in many climates.
Frequently asked questions
What does the 0.623 conversion factor mean in rainwater harvesting calculations?
The factor 0.623 is the number of US gallons produced when one inch of rainfall falls on one square foot of surface. It comes from unit conversion: one inch of water over one square foot equals 0.0833 cubic feet, and one cubic foot holds 7.48 gallons, giving 0.0833 × 7.48 ≈ 0.623. This constant is standard in rainwater harvesting engineering and allows you to work directly with roof area in square feet and rainfall in inches without additional unit conversions.
How does collection efficiency affect how much rainwater I can actually harvest?
Collection efficiency captures all real-world losses between rain falling on the roof and water entering your storage tank. First-flush diverters discard the initial contaminated runoff, roof materials absorb some moisture, gutters lose water to splashing and evaporation, and filters create minor losses. Smooth metal or tile roofs in well-maintained systems typically achieve 85–95% efficiency. Asphalt shingle roofs tend to fall in the 75–85% range, and rooftop gardens or gravel ballast roofs may drop below 70%.
Is harvested rainwater safe to use for vegetable garden irrigation?
Rainwater is generally safe for irrigating vegetable gardens when collected from non-toxic roofing materials like metal, tile, or HDPE-coated surfaces. Avoid using water collected from roofs treated with lead-based paint, copper flashing, or fire-retardant chemicals. A first-flush diverter that discards the first 10–15 gallons of each rain event removes the majority of bird droppings, dust, and other contaminants. For direct contact with edible parts of plants, a basic sediment and carbon filter adds an extra layer of safety.