French Drain Materials
Trench, stone, fabric and pipe quantities for a French drain, with the fall along the run and the flow the perforated pipe can carry at that slope.
Excavation and stone volume with waste, tonnes and truck loads or bags at your density, filter fabric area including the overlap, pipe length, the fall at your slope, and the pipe capacity by Manning.
Example: 100 ft × 12 in × 24 in with a 4 in pipe: 7.41 yd³ of trench, 7.79 yd³ of stone after waste (9 544 kg), 690 sq ft of fabric, 12 in of fall and 5.84 L/s of capacity.
A trench, some stone,
and a pipe with a fall.
How the quantities come out of the trench section, and why the fall gets checked rather than assumed.
The quantities
Everything starts from the trench section: length × width × depth is the excavation, the pipe displaces its own cylindrical volume, any topsoil cover takes the top off, and what is left is stone. Your waste allowance is added to the stone only, since that is the one material that settles, spills and gets left in the truck. Bulk density turns the volume into tonnes and then into bags or truck loads. Fabric area is the wrapped perimeter of the trench plus the overlap percentage you set at the seams and at the top.
Fall and flow
A French drain that does not fall does not drain. The page works out the total fall along the run at the slope you enter and then, separately, how much water the perforated pipe could carry at that slope by the Manning equation. The point is not to size the pipe to a storm — groundwater does not arrive like roof runoff — but to show that a long, flat run has very little capacity and very little margin for a settled low spot. Below about half a percent the page says so.
Limits
No soil classification, no infiltration or permeability rate, no drawdown calculation and no code: this page counts materials and checks a slope. Whether a drain of this size will actually dry the ground depends on the soil, the water table and where the outfall goes, none of which is knowable from a trench section. Stone bulk density, fabric width, pipe size and the Manning n are your inputs from your supplier. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Volumes from the trench section less the pipe; capacity by the Manning equation at your slope and roughness; stone density, fabric width and waste are your inputs; no soil or infiltration assumption
Last reviewed 22 September 2026. How results are checked: How we verify.