How to Use the Rainfall Volume Calculator
A forecast that says "1 inch of rain" doesn't tell you much about how much water will actually run off your roof or yard. This calculator takes your catchment area and rainfall in inches and converts it into gallons of water and its weight in pounds.
The math is straightforward: 1 inch of rain falling on 1 square foot of area yields about 0.623 gallons of water. Multiply that factor by your area and rainfall amount to get total gallons, then multiply gallons by 8.34 (the weight of a gallon of water) to get pounds. This is the same math used to size rain barrels and plan drainage.
The result is a theoretical maximum. Roof material absorption, debris clogging gutters, and overflow before you can collect it all typically mean real-world collection runs about 80-90% of the calculated figure. Also make sure to use your roof's horizontal footprint area — the area it covers when viewed from directly above — rather than the longer sloped surface area.
Frequently Asked Questions
Roof material and drainage losses typically mean actual collection is about 80-90% of the calculated amount. This calculator shows the theoretical maximum.
Enter the horizontal footprint area that catches rain, not the roof's sloped surface area. For a pitched roof, use the area it covers when viewed from directly above.