Rain Barrel & Water Capture
How much water a roof actually catches. Enter the roof footprint and a rainfall, and see the gallons captured, the barrels it would fill, and when to set up and drain it where you live.
Set your place for setup and drain dates.
The capture formula
One inch of rain on one square foot of roof yields about 0.623 gallons, the constant behind every catchment estimate. Multiply your roof's flat footprint, not its sloped surface, by the rainfall in inches and by 0.623, and that is the water reaching your downspouts before losses. Real capture runs a little lower, perhaps eighty to ninety percent, as the first flush wets the roof and some overshoots the gutter. Even a modest roof fills barrels fast, so plan for overflow and route it well away from the foundation.
The 0.623 factor is standard (Utah State University Extension and others). A planning estimate; local rules on rain harvesting vary.