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.
FAQ
How many gallons of water come off a roof in 1 inch of rain?
About 0.623 gallons per square foot of roof footprint. A 1,000 ft² roof sheds about 623 gallons in a 1 inch storm, or about 530 gallons at 85% collection efficiency.
Where does the 0.623 number come from?
One inch of rain on one square foot is 144 cubic inches, and a US gallon is 231 cubic inches. 144 ÷ 231 is about 0.623 gallons.
What collection efficiency should I use?
The calculator starts at 85% to allow for losses before water reaches the tank. Adjust it for your gutters, first-flush diverter and system, and confirm with a qualified pro.
Can I drink the rainwater I collect?
Not without proper treatment, and some local rules restrict it. This calculator is a planning estimate only, not potable-water design.