Rainwater Harvesting Calculator
Rainwater harvesting potential comes down to a clean physical conversion: one inch of rain on one square foot of catchment yields about 0.6234 gallons. This calculator takes your roof footprint in square feet, the rainfall depth in inches, and a runoff coefficient that accounts for real-world losses, then returns the harvestable volume in gallons and liters, plus the theoretical maximum before losses. Because the runoff coefficient depends on your specific roof and system, you enter it, keeping the estimate honest to your setup.
Rainwater harvest formula
Max gallons = area * rainfall inches * 0.6234
Harvestable gallons = max gallons * runoff coefficient
Harvestable liters = harvestable gallons * 3.78541
Lost to runoff = max gallons - harvestable gallons
The factor 0.6234 is exact: one cubic foot is 7.48052 US gallons and an inch is one twelfth of a foot, so 7.48052 divided by 12 is 0.6234 gallons per square foot per inch. The runoff coefficient scales for real-world losses.
Harvesting context
- One cubic foot holds 7.48052 US gallons by the international definition.
- Use the horizontal roof footprint, not the longer sloped area.
- Runoff coefficients for hard roofs are commonly 0.75 to 0.95.
- NOAA publishes local rainfall normals and historical data.
- Local rules and the EPA guide permitted uses and treatment.
Rainwater harvesting: frequently asked questions
How much rainwater can a roof collect?
One inch of rain falling on one square foot of catchment yields about 0.6234 gallons, since one cubic foot holds 7.48052 US gallons and an inch is one twelfth of a foot. Multiply roof area by rainfall depth in inches by 0.6234 and by a runoff coefficient to find the harvestable volume. The conversion is exact.
What is a runoff coefficient?
The runoff coefficient is the fraction of rainfall that actually reaches your tank after losses to splashing, evaporation, first-flush diversion, and absorption. It is typically between 0.75 and 0.95 for a hard roof. Because it depends on your roof and system, you enter it rather than relying on an assumed value.
Should I use roof footprint or sloped area?
Use the horizontal footprint of the roof, that is, the plan-view area the roof covers. Rainfall is measured as depth on a horizontal surface, so the catchment area that matters is the projected footprint, not the longer sloped surface. Enter the footprint in square feet.
How do I find my local rainfall?
The U.S. National Oceanic and Atmospheric Administration publishes historical and normal rainfall data by location. Enter the rainfall depth for the period you care about, whether a single storm, a month, or an annual total, and the calculator scales the harvest accordingly.
Does this guarantee water quality?
No. The calculator estimates collected volume only. Harvested rainwater may need filtration and disinfection before use, and local rules govern permitted uses. Check your state and local regulations and the EPA guidance on rainwater harvesting before using collected water.
Official sources
- U.S. Environmental Protection Agency: Rain barrels and rainwater harvesting.
- U.S. National Oceanic and Atmospheric Administration: U.S. climate normals, precipitation.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.