About this tool
Rainwater Harvesting Calculator is a free, in-browser tool that estimates how much water a roof can collect from a rainfall event. Enter the roof's catchment area (directly or as length × width footprint), the rainfall in inches and a collection efficiency, and it returns the gallons captured plus the barrels needed to store them. Rainfall is entered by you — there is no weather lookup — and nothing is uploaded.
The formula is gallons = rainfall (inches) × roof area (ft²) × 0.623 × efficiency, where 0.623 is the gallons of water that fall on one square foot per inch of rain. Efficiency (0.85 by default) accounts for splashing, first-flush diversion and gutter losses. Barrels needed = ceil(gallons ÷ barrel size), using 55 gallons per barrel by default and rounding up. Liters are gallons × 3.78541.
Use it to size a rain-barrel or cistern system: multiply a typical storm's rainfall by your roof footprint to see the yield, then decide how many barrels or what tank capacity captures it. Even a modest roof collects hundreds of gallons from an inch of rain.