Rainwater Harvesting Calculator
How much rain can the roof actually collect?
Rainwater Harvesting Calculator
Gallons = roof footprint (sq ft) x rainfall (ft) x 7.48052 x runoff coefficient.
Calculated instantly
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What This Calculator Does
The Rainwater Harvesting Calculator estimates how many gallons and litres of rain a roof can deliver in a year, using the roof footprint, annual rainfall, and a runoff coefficient. It is a supply estimate; tank sizing also needs the dry-spell length and how much water you actually use.
How It Works
Rainfall in inches is converted to feet, multiplied by the roof footprint to get cubic feet of rain falling on the plan area, then converted to gallons with 7.48052 gallons per cubic foot. The runoff coefficient reduces that theoretical total for splashing, gutter losses, and first-flush diversion. Litres are gallons multiplied by 3.78541, and the daily average simply spreads the annual total over 365 days.
Example
A 1,500 sq ft roof in a 30 in rainfall area intercepts 1,500 x 2.5 = 3,750 cubic feet of rain. At an 85 percent runoff coefficient, 3,187.5 cubic feet is captured, which is about 23,844 gallons or 90,260 litres a year, or roughly 65 gallons a day averaged across the year.
Formula
roof footprint x rainfall (ft) x runoff coefficientcubic feet x 7.48052gallons x 3.78541annual gallons / 365Reading the Results
Catchable rain per year
A long-run supply estimate after the runoff coefficient. It is not a guarantee for any single year, because rainfall varies widely.
Catchment volume
The same result in cubic feet, useful for comparing with tank volumes and soil storage.
Average per day
The annual total spread evenly. Real supply arrives in bursts, so a tank or cistern is what turns it into a daily resource.
Sources and References
US EPA - Rainwater Harvesting
Catchment, conveyance, and storage components
A general reference for system components and non-potable uses.
Texas A&M AgriLife Extension - Rainwater Harvesting
Catchment area and runoff calculations
Provides the standard catchment formula and practical guidance on first-flush diversion and tank sizing.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Should I use the roof footprint or the sloped roof area?
Use the footprint, meaning the plan area the roof covers. A steep roof has more surface area, but it intercepts the same rainfall per unit of ground area. Using the sloped area overstates the supply.
What runoff coefficient should I use?
A clean metal, tile, or asphalt roof typically loses 10-20 percent to wetting, splash, and first flush, so 80-90 percent is a reasonable range. A green roof, gravel roof, or long gutter run loses more. If you divert the first flush, include that loss here.
How big should my tank be?
Size for the dry spell, not the annual total. Multiply daily demand by the longest typical dry period, then compare with the capture from a typical storm. A tank that holds the whole annual catch would sit empty most of the year and full after one storm.
Can I drink harvested rainwater?
Only with a properly designed treatment train and local approval. Roof runoff can carry bird droppings, metals, and pathogens. Potable use needs first-flush diversion, filtration, disinfection, and testing. For garden use, keep the system simple and avoid stagnant water.
Does the roof material affect water quality?
Yes. Uncoated metal and concrete tile are generally cleaner catchments than old asbestos, lead flashing, or treated wood shingles. Avoid roofs with moss treatments or recent paint, and check local rules before using the water on edible crops.