Published August 18, 2026 · Updated August 23, 2026 · By Tupep
A French drain is a rectangular trench with a round pipe in it, and the pipe takes up room. Most quantity estimates ignore that and over-order stone, then ignore the soil cap on top and over-order again. Both errors run the same direction, so people end up with a leftover pile.
The formula
Gross trench volume (cu ft) = width in ft x depth in ft x length in ft
Pipe displacement (cu ft) = 3.1416 x (pipe outside radius in ft) squared x length in ft
Cap deduction (cu ft) = width in ft x cap thickness in ft x length in ft
Stone needed (cu yd) = (gross minus pipe minus cap) / 27
Then convert to weight. Washed round drainage stone runs about 1.25 tons per cubic yard, washed angular crushed stone closer to 1.35. Wet stone off an open stockpile weighs more, so a ton buys slightly less volume after rain.
Pipe displacement is not trivial
A 4 inch corrugated pipe has an outside diameter of about 4.5 inches, which is 0.375 feet, so a radius of 0.1875 feet.
Cross-sectional area = 3.1416 x 0.1875 x 0.1875 = 0.110 square feet.
Over 60 feet that is 6.6 cubic feet of stone you do not need. Small in absolute terms, but on a 12 inch by 18 inch trench it is more than 7 percent of the total, which is the difference between a clean order and an awkward remainder.
Worked example: 60 ft of 12 in x 18 in trench
- Gross volume: 1.0 x 1.5 x 60 = 90.0 cubic feet
- Pipe displacement: 0.110 x 60 = 6.6 cubic feet
- Soil and sod cap, 3 inches: 1.0 x 0.25 x 60 = 15.0 cubic feet
- Stone: 90.0 – 6.6 – 15.0 = 68.4 cubic feet
- In yards: 68.4 / 27 = 2.53 cubic yards
- In tons at 1.25: 2.53 x 1.25 = 3.17 tons, so order 3.5 tons
Without the pipe and cap deductions the same trench calculates to 90 / 27 = 3.33 cubic yards, or 4.17 tons. That is nearly a ton of stone you would have paid to have dumped on your driveway with nowhere to put it.
Stone per linear foot by trench size
Build the trench section once, then multiply by length. All rows assume 4 inch pipe and no soil cap.
| Trench (W x D) | Gross cu ft per ft | Pipe displacement per ft | Net stone cu ft per ft | Cu yd per 50 ft | Tons per 50 ft |
|---|---|---|---|---|---|
| 8 in x 12 in | 0.67 | 0.11 | 0.56 | 1.03 | 1.29 |
| 12 in x 18 in | 1.50 | 0.11 | 1.39 | 2.57 | 3.22 |
| 12 in x 24 in | 2.00 | 0.11 | 1.89 | 3.50 | 4.38 |
| 18 in x 24 in | 3.00 | 0.11 | 2.89 | 5.35 | 6.69 |
| 24 in x 36 in | 6.00 | 0.11 | 5.89 | 10.91 | 13.63 |
Deduct the cap separately if you are covering the drain with soil. A 3 inch cap on a 12 inch wide trench removes 0.25 cubic feet per linear foot, which is 12.5 cubic feet, or 0.46 cubic yards, per 50 feet.
Sizing the fabric
A properly wrapped French drain is a burrito. Fabric lines the trench, the stone and pipe go inside, and the fabric folds over the top with an overlap. So the fabric width you need is not the trench width.
Fabric width = trench width + (2 x trench depth) + 12 in overlap
For our 12 by 18 trench: 12 + 36 + 12 = 60 inches, so a 5 foot wide roll minimum. Rolls are commonly sold 3 ft, 6 ft, and 12.5 ft wide, so we would buy a 6 foot roll and trim.
Fabric length = trench length x 1.1 to cover end laps and the slack that disappears when you push the fabric into the corners. Sixty feet of trench needs 66 feet of roll.
| Trench (W x D) | Fabric width needed | Roll width to buy |
|---|---|---|
| 8 in x 12 in | 44 in | 4 ft or 6 ft |
| 12 in x 18 in | 60 in | 6 ft |
| 12 in x 24 in | 72 in | 6 ft |
| 18 in x 24 in | 78 in | Wider than 6 ft, or seam two runs |
Material specifics that matter
- Use washed stone, 3/4 inch is the standard. Anything with fines in it will clog the void space and the drain stops draining.
- Angular washed stone holds its voids better under backfill load than rounded stone. Rounded stone is easier to shovel and still works. Either beats dense-graded material, which does not work at all here.
- Void space in clean 3/4 inch stone is roughly a third of the volume, which is where the water actually sits. That is why the stone envelope matters as much as the pipe.
- Non-woven fabric for wrapping, not woven weed barrier. They are different products with different flow rates.
- Set the pipe with the perforations facing down and bed it on 2 to 3 inches of stone rather than on bare trench bottom. Add that bedding depth into your trench depth before calculating.
Ordering notes
Stone yards sell by the ton for anything over half a yard and by the bag below that. For runs under about 25 feet, bagged 0.5 cubic foot stone is often simpler despite the higher unit price. Past that, we order bulk and dump it as close to the trench line as the truck can reach, because a cubic yard of wet stone is roughly 2,500 pounds and every extra foot of wheelbarrow run is real work.
Related guides
- How Many Tons of Gravel for a 20×40 Driveway
- Pea Gravel vs Crushed Stone
- How Many Bags of Concrete for a 10×12 Shed Slab
- Bags vs Bulk: The Break-Even Is 8 Yards
Sources and Further Reading
Run your own numbers with the Gravel Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
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