Rainfall Collection Calculator
Estimate gallons of rainwater collected from roof catchment area, rainfall amount, and collection efficiency.
sq ft
in
%
Accounts for overflow, splash loss, and first-flush diversion
Result
Estimated Gallons Collected
529.5
Theoretical Maximum (100% efficiency)
623.0 gal
How to Use the Rainfall Collection Calculator
Enter your roof's catchment area in square feet and the amount of rainfall in inches to estimate how many gallons of water you could collect. Every square foot of roof catches about 0.623 gallons per inch of rain that falls on it. A collection efficiency percentage (default 85%) accounts for real-world losses — water that overflows the gutters, splashes off the roof, or is diverted by a first-flush system that discards the initial, dirtiest runoff.
Example
A 1,000 sq ft roof catches 1 inch of rain: theoretical collection is 1,000 × 1 × 0.623 = 623 gallons. At an 85% collection efficiency, the actual amount captured is about 623 × 0.85 = 529.6 gallons.
Common Use Cases
- Sizing a rain barrel or cistern based on typical local rainfall and roof size.
- Estimating how much municipal water use a rainwater harvesting system could offset.
- Planning irrigation capacity for a garden fed by collected rainwater.
FAQs
- Where does the 0.623 conversion factor come from? One inch of rain falling on one square foot of surface equals 0.623 gallons of water — it's a standard conversion used throughout rainwater harvesting calculations based on the volume of a one-inch-deep layer of water over one square foot.
- Why isn't collection efficiency 100%? Real systems lose some water to gutter overflow during heavy rain, evaporation, splashing, debris blockage, and first-flush diverters that intentionally discard the initial runoff carrying roof debris and contaminants. 80-90% is a commonly used realistic range.
- Does roof material affect how much I can collect? Yes — smooth, non-porous materials like metal roofing collect more efficiently than porous materials like some shingles or wood shakes, which absorb a small amount of water before runoff begins.