Dry Well Calculator

Overview

Estimates runoff volume from drainage area and rainfall, then relates it to stone void storage and an entered soil percolation rate.

Assumptions

• Roof/paved runoff coefficient is 0.95 and stone void ratio is 0.40.
• Percolation is a user planning input, not a field test result.

Worked example

For 1,000 sq ft and 1 in rain, runoff is 1,000 × 1 ÷ 12 × .95 = 79.2 cu ft; stone volume at 40% voids is about 198 cu ft.

Common mistakes

• Using a soil map value as a substitute for site testing.
• Installing near a foundation, well, or poorly drained area without review.

Limitations

Groundwater, seasonal saturation, overflow routing, and local design criteria are not assessed.

Safety and local requirements

Check local permits, setbacks, and utility locations; do not enter an excavation or dry well structure.

Sources

• EPA — Soak Up the Rain
• OSHA — Landscaping and Horticulture