Driveway rainfall runoff calculator (asphalt pavement)

Runoff volume, edge wash, and soft spots

A 50×10 ft asphalt pad in a 1″ storm can shed on the order of ~280 gallons at a 0.90 planning coefficient. That water has to go somewhere. If it sheets along the edge or ponds in a low spot, it undercuts the shoulder and saturates the base — a common path to the failures covered in asphalt driveway soft spots and asphalt driveway drainage.

Formula: Gallons ≈ area (ft²) × rainfall (inches) × 0.623 × C. Constant 0.623 ≈ 144 ÷ 231. Coefficients above are coarse planning labels only (asphalt/concrete ~0.90, gravel ~0.50, lawn ~0.20). Real runoff depends on slope, texture, cracks, antecedent moisture, and storm intensity.

Note: This estimates surface runoff volume for a storm depth, not pipe/gutter capacity, inlet spacing, or French-drain trench sizing. Use it to talk volumes with a contractor, then confirm design locally.

Related: rainfall roof runoff calculator · asphalt tonnage calculator · driveway asphalt 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.