Drainage & Runoff Calculator | LandscapeCalculating.com

Drainage & Runoff CalculatorRunoff volume, French drain stone and dry-well sizing for a design storm.Free drainage & runoff calculator with formulas, waste factors, cost estimates and professional guidance.Formula: gallons = area × (rainfall in ÷ 12) × runoff coefficient × 7.48How to use this calculator

Identify contributing surfaces and their runoff coefficients before sizing anything.

Pitch solid discharge pipe at 1–2% and daylight it where water can safely leave.

Percolation-test before specifying a dry well and always design an overflow path.

Frequently asked questions

How much water comes off a roof in a 1 in storm?

About 623 gallons per 1,000 sq ft of roof at a 0.95 runoff coefficient.

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