Rainwater Harvesting Calculator
Estimate how much rainwater you can capture from a roof, given its area, your annual rainfall, and collection efficiency. In gallons and liters. Runs in your browser.
Accounts for first-flush diversion, splash, and evaporation. ~80–90% is typical.
Water you could capture
- Annual harvest (gallons)
- 15,896 gal
- Annual harvest (liters)
- 60,173 L
- Per inch of rain
- 530 gal
Based on the standard rule that 1 inch of rain on 1 ft² yields 0.623 gallons (1 mm on 1 m² = 1 L). Catchment area is the roof's horizontal footprint, not the sloped surface. Check local regulations — rainwater harvesting is restricted in some areas.
About this tool
A surprising amount of water falls on a roof. This calculator estimates how much you could collect from the catchment area, your local annual rainfall, and a collection-efficiency factor. It rests on a simple, exact relationship: one inch of rain falling on one square foot of horizontal area is 0.623 US gallons (equivalently, one millimeter on one square meter is exactly one liter). Multiply by your roof's footprint and yearly rainfall and you get the gross volume; the efficiency slider then trims it for real-world losses — first-flush diversion that discards the dirty initial runoff, splash, overflow, and evaporation — which typically leave 80–90% usable. A key subtlety the tool notes: the catchment area is the roof's flat footprint (the area it covers on the ground), not the longer sloped surface, because rain is measured on the horizontal. The result helps size a tank or judge how much irrigation or non-potable demand a system could meet. Local rules vary — some places restrict or regulate harvesting — so check before building. Everything runs in your browser.
How to use it
- Enter your roof's catchment footprint (ft² or m²).
- Enter your area's average annual rainfall (inches or mm).
- Set a collection efficiency — about 85% is typical.
- Read the gallons and liters you could capture per year.
Frequently asked questions
- How much water does rain on a roof produce?
- One inch of rain on one square foot yields 0.623 US gallons; in metric, one millimeter on one square meter is exactly one liter. So a 1,000 ft² roof in a 30-inch rainfall year produces about 18,700 gallons gross, before efficiency losses.
- Do I use the sloped roof area or the footprint?
- The footprint — the horizontal area the roof covers. Rainfall is measured as depth on a level surface, so a steeper roof of the same footprint catches the same rain; only the ground area it shelters matters for the volume.
- What does collection efficiency account for?
- Real losses: first-flush diverters that dump the initial dirty runoff, water that splashes or overshoots the gutters, overflow when the tank is full, and evaporation. 80–90% captured is a common assumption; rougher systems are lower.
- Is harvested rainwater safe to drink?
- Not without treatment. Roof runoff carries dust, bird droppings, and debris. It is well suited to irrigation, flushing, and other non-potable uses; potable use requires filtration and disinfection and may be regulated.
- Is rainwater harvesting legal everywhere?
- No — rules vary by country, state, and city, from encouraged (with rebates) to restricted or permit-required. Check your local water authority's regulations before installing a system.
- Is anything uploaded?
- No. The calculation runs entirely in your browser with no network request.