French drain gravel volume
gravel = trench volume − pipe volume + allowanceIrregular trenches and unstable soils often require more stone than a clean rectangular calculation.
French drain calculator
Estimate washed drainage stone, perforated pipe, and filter fabric for a rectangular trench.
Enter the finished trench dimensions
The gravel result subtracts the pipe displacement and adds the selected order allowance.
The outlet controls the system
A French drain cannot move water uphill. Confirm continuous fall, a legal discharge point, suitable stone and fabric, maintenance access, and protection against sediment.
Worked example
A 50 ft drain, 12 in wide and 18 in deep
Trench50 × 1 × 1.5 = 75.00 ft³
4 in pipeπ × (4 ÷ 24)² × 50 = 4.36 ft³ displaced
Gravel + 10%(75.00 − 4.36) × 1.10 = 77.70 ft³, or 2.88 yd³
Pipe and fabric55 ft of pipe and about 275 ft² of fabric with the built-in allowances
What the estimate includes
- Stone filling the entered trench dimensions around one round pipe.
- Ten percent extra pipe for cuts and connections.
- Fabric for the bottom, two sides, 12 inches of overlap, and ten percent for cutting.
What to verify before ordering
- The trench can maintain fall to a lawful outlet.
- The specified stone is clean, washed, and compatible with the chosen fabric.
- Cleanouts, fittings, catch basins, and solid outlet pipe are counted separately.
- Utility locations and safe excavation requirements are known.