Enter your roof footprint and a rainfall amount to see how many gallons you can actually collect — and how many rain barrels that fills.
How much water a roof really sheds
Roofs are astonishing catchments hiding in plain sight: every inch of rain on every square foot sheds 0.623 gallons. Enter the roof’s footprint (the ground area it covers — slope doesn’t change the catch), a rainfall amount, and a collection efficiency for gutter losses and first-flush diversion, and this calculator returns the gallons — and the number of standard 55-gallon barrels they fill, which is usually the surprise.
How to use it
- Enter the roof footprint in square feet (or just the section feeding your barrels).
- Enter a rainfall amount — one storm, or a month’s total.
- Keep ~85% efficiency unless your setup suggests otherwise.
The formula
A worked example
A 1,000 sq ft roof in one inch of rain sheds 623 gallons — at 85% collection, about 530 gallons, or 9–10 rain barrels. That single number explains the universal first-timer experience: one barrel under one downspout overflows in the first twenty minutes of a real storm. Sizing storage to your goals — and your region’s rules, since a few places regulate collection — starts with this math.