French Drain Calculator
Calculate gravel volume and perforated pipe length for a French drain installation.
Formula
Volume (cu yd) = L × W × D ÷ 27; Slope: 1% minimum (1 in drop per 8.3 ft)
- Minimum slope: 1% (1 in of drop per 8.33 ft of run).
- Pipe: 4 in perforated pipe wrapped in filter sock.
- Gravel: clean washed stone, 3/4–1.5 in, wrapped in geotextile fabric.
- Trench depth: pipe invert should be below frost line.
40 ft drain, 1.5 ft wide, 2 ft deep, 1% slope
Inputs
- Drain Length: 40 ft
- Trench Width: 1.5 ft
- Trench Depth: 2 ft
- Slope: 1 % (in/ft)
40×1.5×2=120 cu ft ÷ 27 = 4.44 cu yd. Drop: 1%×40 ft×12 = 4.8 in.
Frequently asked questions
How deep should a French drain be?
6–18 inches below the lowest point to be drained, and below frost line for year-round function. Typical residential installations: 18–24 in deep.