Stormwater
Roof Runoff and Rainwater Harvesting Calculator
One inch of rain on one square foot is 0.623 gallons. Enter the catchment area, rainfall depth and collection efficiency to size a rain barrel, cistern or drywell.
Runoff collected gal
Runoff collected ft³
Formula
Volume (ft³) = area × rainfall (in) ÷ 12 × efficiency. Gallons = ft³ × 7.48052. Combined: 1 in on 1 ft² = 0.6234 gal.
Worked example
A 1,500 ft² roof footprint in a 1 in rain at 80% efficiency: 1,500 × 1 ÷ 12 × 7.48052 × 0.80 = 748.1 gal (100.0 ft³).
Assumptions and limits
- Area is the horizontal projection, not the sloped roof surface.
- Efficiency accounts for evaporation, first flush diversion and gutter overflow; 75% to 90% is a common range for roofs.
Frequently asked questions
How much water comes off a roof in an inch of rain?
About 623 gallons per 1,000 ft² of footprint before losses.
Is rainwater harvesting legal?
It is legal in most states, though a few regulate volumes or require permits. Check your state water rights office.
How big should a drywell be?
It must store the design storm runoff that cannot infiltrate during the storm. Local codes often specify the design storm and require a percolation test.