French Drain Calculator

Overview

Turns trench length, width, and depth into gravel volume, pipe length, fabric wrap area, and fittings.

Assumptions

• The trench is rectangular.
• Gravel is based on full trench volume; the pipe displacement is not subtracted.

Worked example

A 30-ft trench, 18 in deep and 12 in wide is 45 cu ft = 1.67 cu yd; at 1.4 tons/yd it is 2.33 tons.

Common mistakes

• Confusing a French drain with a solid-pipe roof drain.
• Installing without a legal, functional outlet.

Limitations

It does not model watershed flow, soil permeability, groundwater, or outlet capacity.

Safety and local requirements

Call 811 before trenching and keep excavations safe. Check local stormwater and discharge requirements.

Sources

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