Rainwater Harvesting Calculator

Even a modest roof collects a surprising amount of water. Enter your roof’s footprint, your annual rainfall and your roof material, and the calculator estimates how much water you can harvest each year, how much of your household use it could cover, and a sensible tank size.

Estimates for planning only. Always check equipment specifications, local codes and, for electrical work, a licensed professional.

The formula behind it

One inch of rain falling on one square foot of roof produces about 0.623 US gallons of water (1 mm on 1 m² produces 1 liter). Not all of it reaches the tank: some splashes off, evaporates or is diverted by the first-flush device that keeps the dirtiest water out. The runoff coefficient accounts for this — about 0.95 for metal roofs and lower for rougher surfaces.

Collected water = roof area × rainfall × 0.623 × runoff coefficient

Sizing your tank

Annual totals hide big seasonal swings. If your rain falls mostly in winter and you need water in summer, your tank must hold several months of use. As a starting point, the calculator suggests about six weeks of average collection; increase it if you have a long dry season.

Before you build

• Check local rules. Rainwater collection is legal in most places, but some U.S. states limit volumes or require permits. See our off-grid laws by state overview.
• Treat before drinking. Roof water can carry bacteria, bird droppings and debris. Use a first-flush diverter, screened gutters and proper filtration and disinfection for any water you drink.
• Use the right materials. Choose opaque, food-grade tanks and keep them covered to stop algae and mosquitoes.

Ready to build? Read how to set up a rainwater collection system.

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