A french drain calculator estimates the gravel, perforated pipe, and landscape fabric you need to build a drainage trench. Enter your trench dimensions for accurate material quantities.
Trench Dimensions
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How to Calculate French Drain Materials
A french drain trench is typically filled with 2-3 inches of gravel below the pipe, the perforated pipe wrapped in fabric, then gravel to within 2-3 inches of the surface, with topsoil or sod on top. This calculator accounts for that volume.
Gravel Calculation
Calculate trench volume (length × width × depth), subtract the pipe volume (negligible for 4-inch pipe), and convert to tons or cubic yards. One cubic yard of 3/4-inch washed stone weighs approximately 1.3 tons.
Estimates are approximate. Verify quantities with your supplier before ordering.
Frequently Asked Questions
What is a french drain?
A french drain is a gravel-filled trench with a perforated pipe that redirects surface and groundwater away from problem areas. It relieves hydrostatic pressure against foundations and prevents waterlogging in low-lying areas.
How deep should a french drain be?
For surface drainage, 8-12 inches deep is usually sufficient. For foundation drainage, dig to the base of the foundation footer, typically 18-24 inches or deeper. The pipe should slope 1 inch per 8 feet toward the outlet.
What size perforated pipe is standard?
4-inch diameter corrugated perforated pipe is the standard for residential french drains. Use solid pipe at the outlet end. For high-flow applications or clay soils, use 6-inch pipe.
What type of gravel is best?
Use clean 3/4-inch washed stone or pea gravel. Avoid crushed stone with fines (powder) that can clog the perforated pipe. Wrap the pipe in filter fabric sock to prevent sediment migration.
Is this calculator free?
Yes, completely free with no signup required.