Rainwater Harvest Calculator: Gallons & Cubic Feet

Every storm drops a predictable amount of water on a roof, and the volume you can catch depends on only two things: how big the roof footprint is and how much rain falls. The calculator multiplies the catchment area by the rainfall depth to get a raw volume, then trims it by a collection efficiency to account for water that splashes out of gutters, wets a dry roof, or is diverted by a first-flush device.

The imperial shortcut hidden inside the math is that one inch of rain on one square foot of roof yields about 0.623 gallons. That number is pure geometry: an inch of depth over a square foot is one-twelfth of a cubic foot, and a cubic foot holds 7.48 gallons. Measure the roof by its footprint — the flat area it shades on the ground — not its sloped surface, because a pitched roof sheds the same rain as its shadow.

Efficiency is where real roofs differ from the ideal. A smooth metal roof gives back nearly everything, while asphalt shingles, debris screens, and the first few gallons that rinse the roof clean pull a typical system down to roughly 85%. Lowering the efficiency is the honest way to keep the estimate from overpromising.