Driveway rainfall runoff calculator (asphalt pavement)
Estimate surface runoff gallons for a driveway storm event. Formula: area × rainfall inches × 0.623 × runoff coefficient. Planning surface-volume sketch only — not pipe, gutter, or French-drain hydraulics design.
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.
FAQ
How much water runs off a driveway in 1 inch of rain?
Multiply area (ft²) × rainfall (inches) × 0.623 × the runoff coefficient. A 50 × 10 ft asphalt driveway at 0.90 sheds about 280 gallons in a 1 inch storm.
What runoff coefficient should I use for asphalt?
About 0.90 is a typical planning value for asphalt or concrete, 0.85 for aged or textured asphalt, 0.50 for gravel and 0.20 for lawn. Real runoff depends on slope, texture, cracks, soil moisture and storm intensity.
Can this size a French drain or gutter?
No. It estimates surface runoff volume for a storm depth, not pipe or gutter capacity, inlet spacing or French drain trench sizing. Use it to talk volumes with a contractor.
Why does driveway runoff matter?
Water that sheets along the edge or ponds in a low spot undercuts the shoulder and saturates the base, a common path to soft spots and edge failure.