Rainwater Harvest Calculator
Rainwater harvesting can significantly reduce irrigation water demand and lower utility bills. The standard formula used by USDA and EPA rainwater harvesting guides multiplies roof area in square feet by rainfall depth in inches by 0.623 to convert to US gallons. A collection efficiency factor accounts for losses at the roof surface, gutters, and first-flush diverters. This calculator gives both the gross potential and the net collectable volume after efficiency losses. Annual potential is calculated from average annual rainfall and can help you size your storage tank.
Rainwater collection formula
Gross gallons = Area (sq ft) x Rainfall (in) x 0.623
Net gallons = Gross gallons x (Efficiency% / 100)
The factor 0.623 is derived from the unit conversion: 1 sq ft x 1 in depth = 144 cu in = 0.6234 US gallons (using 231 cu in per US gallon). Efficiency of 80 to 90 percent is typical for well-maintained metal or asphalt roof systems.
Rainwater harvesting planning tips
- Use average monthly rainfall data from NOAA for your location to estimate seasonal yield.
- Size your tank to capture the runoff from the largest typical storm event in your area.
- Install first-flush diverters (1 gal per 100 sq ft of roof) to improve water quality.
- Use food-grade polyethylene tanks or food-grade-lined metal tanks for water used on edible crops.
- Check state law: Colorado limited residential rainwater collection to 110 gallons per household until 2016 law changes; verify current rules in your state.
Rainwater harvest calculator: frequently asked questions
How do I calculate rainwater collection from a roof?
Multiply the roof footprint area (in sq ft) by the rainfall depth (in inches) and by 0.623. For example, a 1,000 sq ft roof receiving 1 inch of rain: 1,000 x 1 x 0.623 = 623 gallons. The factor 0.623 accounts for unit conversion (1 sq ft x 1 inch = 0.6234 US gallons).
What is the collection efficiency factor?
Not all rainfall that hits a roof reaches your tank. Losses occur to evaporation, first-flush diverters, overflow, and surface absorption. Typical collection efficiency is 80 to 90 percent for metal roofs, 75 to 85 percent for asphalt shingles. Apply a factor of 0.80 to 0.90 to the gross calculation.
Is rainwater collection legal in the US?
Rainwater harvesting laws vary by state. Most western states now permit it; some (Colorado, historically) had restrictions that have since been relaxed. USDA and EPA do not regulate residential rainwater collection for outdoor use. Always check your state's water rights laws before installing a system.
What size tank do I need?
Tank sizing depends on your dry-season length and demand. A common rule of thumb is to size the tank to capture 2 to 4 weeks of irrigation demand. If your garden needs 200 gallons per week and you have a 3-week dry period, you need 400 to 800 gallons of storage capacity.
How does first-flush diversion affect collection?
First-flush diverters discard the first 10 to 20 gallons (often 1 gallon per 100 sq ft of roof) to wash away bird droppings and other contaminants before rain enters the tank. Account for this loss by subtracting the first-flush volume from each rainfall event's total.
Official sources
- EPA WaterSense, rainwater harvesting: epa.gov.
- USDA NRCS, water harvesting and storage: nrcs.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.