Rainwater Harvesting Calculator
Estimate how many gallons of rainwater your roof can collect each year and size your storage tank accordingly. Ideal for homeowners planning irrigation systems, off-grid setups, or drought resilience.
About this calculator
Rainwater harvesting potential depends on four factors: the roof's catchment area, local annual rainfall, a runoff coefficient for the roof material (accounting for evaporation, splash loss, and absorption), and a storage goal fraction. The standard volumetric formula converts rainfall depth over an area into gallons: Gallons = roofArea (sq ft) × rainfall (inches) × 0.623 × roofCoefficient × storageGoal. The constant 0.623 converts the product of square feet and inches into US gallons (1 inch of rain on 1 sq ft = 0.623 gallons). The roofMaterial coefficient typically ranges from 0.75 for gravel or green roofs to 0.95 for metal roofing, reflecting how cleanly each surface sheds water. The storageGoal fraction (e.g., 0.5 to capture half of annual yield) lets you size a practical tank rather than one large enough for the entire year's supply.
How to use
Assume a 1,500 sq ft metal roof (roofMaterial coefficient = 0.90), in a region with 30 inches of annual rainfall, targeting storage for 25% of annual yield (storageGoal = 0.25). Calculation: 1,500 × 30 × 0.90 × 0.623 × 0.25 = 1,500 × 30 = 45,000; × 0.90 = 40,500; × 0.623 = 25,231.5; × 0.25 = 6,308 gallons. Round to 6,308 gallons — so a pair of 3,200-gallon poly tanks would comfortably meet this storage target. At typical irrigation use of 1 gallon per sq ft per week, this volume supports roughly 6,300 sq ft of garden for one week.
Frequently asked questions
How many gallons of rainwater can I collect per inch of rainfall from my roof?
The rule of thumb is approximately 0.623 gallons per square foot of roof area per inch of rainfall, before accounting for losses. For a 1,000 sq ft roof, one inch of rain theoretically yields about 623 gallons. In practice, your roof material's runoff coefficient reduces this: a tile roof at 0.80 efficiency yields around 498 gallons per inch, while a smooth metal roof at 0.95 yields about 592 gallons. First-flush diverters, which discard the initial dirty runoff, can reduce usable yield by a further 5–10% but greatly improve water quality.
What size rainwater storage tank do I need for a typical residential property?
Tank sizing depends on the gap between your collection events — how many dry days you need to bridge — and your daily demand. A household using harvested water only for outdoor irrigation might need 500–2,500 gallons, while an off-grid home relying on rainwater for all uses may require 10,000 gallons or more. A good starting point is to calculate your peak monthly demand, estimate how much rain falls in your driest two months, and size the tank to cover the shortfall. Modular poly tanks are easiest for residential installs; underground cisterns save space and keep water cooler and algae-free.
Does roof material type significantly affect rainwater collection efficiency and water quality?
Yes, both significantly. Metal and tile roofs have runoff coefficients of 0.90–0.95, meaning very little water is lost to absorption or splash. Asphalt shingles run 0.85–0.90 but can leach trace chemicals and granules, making first-flush diversion especially important. Green or gravel roofs have coefficients as low as 0.50–0.70 because the growing medium absorbs substantial rainfall. For potable or near-potable uses (toilet flushing, laundry), treated metal or tile roofs with smooth gutters provide the cleanest source; asphalt shingle water should generally be restricted to irrigation without additional filtration.