Rainfall roof runoff calculator (rainwater gallons)
Estimate how many gallons of rain leave a roof footprint in a given storm. Uses area × rainfall inches × 0.623 × collection efficiency. Optional tank capacity shows captured vs overflow. Planning only — not plumbing, stormwater, or potable-water design.
Why roof runoff matters for asphalt driveways
Downspouts that dump onto or beside an asphalt edge keep the shoulder wet. That moisture softens the base, encourages edge ravel, and feeds the same failure modes described in our asphalt driveway soft spots guide. Knowing gallons for a 1″ storm helps you size barrels, plan first-flush diversion, or sketch whether a French drain is even in the right ballpark — then confirm with a qualified pro.
Formula: Gallons ≈ area (ft²) × rainfall (inches) × 0.623 × (efficiency ÷ 100). The constant 0.623 ≈ 144 ÷ 231 (cubic inches of 1″ rain on 1 ft² ÷ cubic inches in a US gallon). After efficiency, captured = min(gallons, tank); overflow = max(0, gallons − tank).
Related: driveway rainfall runoff calculator · asphalt driveway drainage · asphalt tonnage calculator.
Disclaimer: Planning estimate only. Uses area × rainfall inches × 0.623 × a runoff/collection factor. Not a licensed engineering, stormwater, plumbing, or potable-water design. Local codes, soil, slope, first-flush, and system losses vary — confirm with a qualified pro before installing drainage or rainwater systems. Rainwater for drinking requires proper treatment and may be restricted by local rules.