Quick Answer — How Much Rain Can a Roof Actually Collect?
The standard rainwater harvesting formula is: Gallons = Roof Area (sq ft) × Rainfall (inches) × 0.623 × Runoff Coefficient × System Efficiency. The 0.623 constant comes from the physical fact that 1 inch of rain falling on 1 square foot of catchment yields 0.623 gallons of water, before any losses.
Not all of that water reaches your tank. A runoff coefficient accounts for the roof surface: smooth metal roofing sheds about 95% of what falls on it, asphalt shingle or concrete sheds about 90%, and rougher built-up roofing like tar-and-gravel sheds closer to 80% because more water is absorbed or evaporates before reaching the gutter. A system efficiency factor then accounts for first-flush diverters, gutter overflow during heavy storms, and filter losses — commonly 75-90% for a well-maintained system.
This calculator applies both factors to your roof's catchment area and local annual rainfall to estimate realistic annual and monthly harvest volume, then sizes a tank against your actual daily water demand.