French Drain Calculator — Gravel, Pipe & Fabric
Plan a French drain end to end. Enter the trench dimensions and this calculator returns the drainage gravel in tons, the length of perforated pipe, and the landscape fabric to line and wrap the trench.
How to use this calculator
- Enter the trench length, and its width and depth in inches.
- A 12 in wide, 18 in deep trench around a 4 in perforated pipe is typical for yard drainage.
- Line the trench with fabric, add a gravel bed, lay pipe (holes down), backfill with gravel, then wrap the fabric over the top.
- Slope the trench at least 1% (1 in per 8 ft) so water flows to the outlet.
The formula
A French drain needs gravel to fill the trench, pipe to carry water, and fabric to keep soil out of the gravel.
Gravel (cubic feet) = length × width (in) ÷ 12 × depth (in) ÷ 12. Tons = cubic feet ÷ 27 × 1.4.
Pipe ≈ trench length + 5%. Fabric = length × (width + 2 × depth) ÷ 12, plus ~15% for overlap, since it lines the bottom, sides, and folds over the top.
Worked example
A 40 ft trench, 12 in wide, 18 in deep.
Gravel = 40 × (12 ÷ 12) × (18 ÷ 12) = 60 cu ftTons = 60 ÷ 27 × 1.4 ≈ 3.1 tonsPipe = 40 × 1.05 = 42 ftFabric = 40 × ((12 + 36) ÷ 12) × 1.15 ≈ 184 sq ft
This drain needs about 3.1 tons of gravel, 42 ft of pipe, and ~184 sq ft of fabric.
Frequently asked questions
How much gravel for a French drain?
A 40 ft trench 12 in wide and 18 in deep holds about 60 cubic feet of gravel — roughly 3 tons. Use clean, washed drainage stone (often #57) so it does not clog.
Do I need landscape fabric in a French drain?
Yes. Wrapping the gravel in permeable fabric keeps fine soil from migrating in and clogging the stone, which is the main reason French drains fail over time.
What slope does a French drain need?
Aim for at least a 1% slope — about 1 inch of drop for every 8 feet of run — so water reliably flows toward the outlet.