Rainwater Collection Calculator — Roof Yield and Storage
Estimate how many gallons a roof catchment can supply during a rain event and how much of that yield a tank can accept.
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View all saved project estimatesWhat does the rainwater collection calculator calculate?
One inch of rain on one square foot equals about 0.623 gallon. Multiply roof footprint by rainfall, then apply runoff and collection efficiency. Storage limits the captured amount. This is a planning estimate; potable use, overflow routing, mosquito control, roof materials, and permits need local verification.
Roof rainwater yield formula
The 0.623 factor converts one inch of water over one square foot to U.S. gallons. Separate coefficients account for roof runoff losses and collection-system losses before comparing the event yield with empty tank capacity.
How to measure for an accurate estimate
- Measure the horizontal roof footprint feeding the selected gutters, not the sloped roof surface area.
- Divide complex roofs into rectangles and include only sections connected to the planned storage system.
- Use a local rainfall depth for the design event or period being evaluated, not an annual total by default.
- Subtract water already in the tank from total capacity before entering the available storage value.
One-inch rain on a garage roof
A 40 ft by 30 ft roof receives 1 in of rain with 90% runoff, 90% efficiency, and 500 gal of empty storage.
Potential yield is about 606 gal; the tank captures 500 gal and roughly 106 gal requires managed overflow.Before you purchase material
- Choose opaque, screened storage designed to exclude sunlight, debris, animals, and mosquitoes.
- Size gutters, downspouts, first-flush devices, overflow, and erosion protection for peak flow as well as volume.
- Verify that the roof, coating, fittings, and tank are suitable for the intended nonpotable or potable use.
- Provide a stable foundation and protected overflow path before filling a large tank that becomes extremely heavy.
Common questions
How many gallons can one inch of rain produce?
One inch falling on one square foot is approximately 0.623 U.S. gallon. Therefore, a 1,000-square-foot catchment has a theoretical yield near 623 gallons. Real collection is lower because of roof wetting, first flush, gutter loss, leaks, splash, and system inefficiency.
Should I use roof area or roof footprint?
Use the horizontal footprint of the roof sections draining to the collection system. Rainfall depth is measured on a horizontal plane, so using sloped surface area overstates yield. Split hips, valleys, additions, and disconnected gutter sections into separate footprint shapes when necessary.
Can collected rainwater be used for drinking?
Do not assume roof runoff is potable. Drinking-water use can require approved roofing materials, treatment, testing, cross-connection protection, permits, and ongoing maintenance. Requirements vary widely, so consult local health and plumbing authorities before designing any system for consumption or indoor connections.
How large should a rainwater tank be?
Tank sizing should balance catchment yield, local rainfall timing, intended demand, available space, structural support, budget, and overflow frequency. Event yield is only one input. A monthly water-balance analysis provides a better storage decision where irrigation reliability or drought carryover matters.
Sources and reference standards
- USDA Natural Resources Conservation Service — Rainwater HarvestingFederal planning reference for estimating, collecting, storing, and using harvested rainwater.
Use this result in a complete project plan
Yard drainage planner
A wet yard is a water-path problem before it is a gravel, pipe or pavement problem. Roof discharge, neighboring grades, compacted soil, irrigation leaks and high groundwater can create similar symptoms but require different responses. This guide organizes observations, elevations, contributing areas, outlet review and material estimates without presenting a generic trench as a universal drainage design.
Rainwater-to-garden planner
A rain barrel or cistern can fill quickly while a garden needs water across many dry days, so one storm-yield number cannot answer whether storage will be useful. This guide connects roof catchment, tank capacity, measured irrigation-zone demand and planting area while keeping potable use, plumbing protection, structural support, overflow and local rules outside the arithmetic.
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