Drainage
How Much Gravel Do You Need for a French Drain?
Calculate French drain gravel in cubic yards and tons: the trench-minus-pipe formula, per-foot quick table, the right stone spec, and a 50-ft worked example.
Gravel is the French drain's biggest material line - typically 60–70% of the bill - and the one most often under-ordered because people forget the pipe displaces so little and the trench swallows so much. The math is simple; the material spec is where drains live or die.
The formula
Pipe (ft³) = π × (dia/24)² × Length
Stone = (Trench − Pipe) × 1.05 waste → tons = yd³ × 1.2
Washed open-graded stone runs light - about 2,400 lb (1.2 tons) per loose cubic yard, because the voids that make it drain also make it weigh less than dense-graded products.
Quick table: stone per linear foot
| Trench (W × D) | Pipe | Stone per ft | Feet per ton | Tons per 50 ft |
|---|---|---|---|---|
| 6 × 12 in | 3 in | 0.47 ft³ | ~47 ft | 1.1 |
| 12 × 18 in | 4 in | 1.42 ft³ | ~15 ft | 3.3 |
| 12 × 24 in | 4 in | 1.92 ft³ | ~11.5 ft | 4.4 |
| 18 × 24 in | 4 in | 2.92 ft³ | ~7.6 ft | 6.6 |
| 18 × 30 in | 6 in | 3.55 ft³ | ~6.2 ft | 8.1 |
Includes 5% waste; assumes 1.2 t/yd³ washed stone. Deeper curtain drains scale up fast - depth is the cost driver.
Worked example: 50-ft drain, 12 × 18 in trench, 4-in pipe
- Trench: 50 × 1.0 × 1.5 = 75 ft³
- Pipe displacement: 3.1416 × (4/24)² × 50 = 4.4 ft³
- Stone: (75 − 4.4) × 1.05 = 74.1 ft³ = 2.75 yd³
- Tons: 2.75 × 1.2 = 3.3 tons - one small dump delivery
- Sanity check with the table: 50 ft ÷ 15 ft/ton = 3.3 ✓
The French drain calculator runs this plus pipe sections, fabric area, and required drop in one pass.
Open the French Drain CalculatorThe stone spec - read this twice
Washed: rinsed at the quarry so no dust coats the particles. Angular: crushed faces lock in place; rounded stone migrates. 3/4–1.5 in: big enough for large voids, small enough to bed a pipe. No fines: the entire function of the drain is the void space between stones - stone dust fills those voids permanently. If the supplier says "crusher run is cheaper," that's true, and it will also turn your drain into a buried concrete-ish strip within a few seasons. The right ask: "washed #57 drainage stone" (or your region's equivalent).
Ordering and handling notes
Stone arrives in one pile, and a 3-ton pile is about a full pickup-bed times four in wheelbarrow trips - plan the dump location along the trench line if truck access allows. Keep the stone on a tarp, not the lawn; recovering the last half ton out of grass wastes an hour and a rake. If the job needs both drainage stone and base gravel for other work, order separately - never substitute one for the other in either direction.
When to call a professional
Drains deeper than 3–4 ft, under driveways, near foundations, or in slopes need proper bedding, compaction around the works, and sometimes engineered filter design (matching fabric and stone to the soil gradation). If the drain protects a structure rather than a lawn, have the design checked before the stone is ordered.
Frequently Asked Questions
How much gravel per foot of French drain?
A standard 12 x 18 in trench with 4-in pipe needs about 1.4 cubic feet of stone per linear foot - roughly 1 ton per 15 feet of drain. A 50-ft drain takes about 3.3 tons.
What kind of gravel goes in a French drain?
Washed, angular stone between 3/4 and 1.5 inches with no fines - commonly sold as #57 washed stone or drainage rock. Never crusher run, screenings, or anything 'with dust': fines clog the voids that make the drain work.
Can I use pea gravel for a French drain?
It drains, but rounded pea gravel migrates, packs tighter over time, and gives less void space than angular stone. If angular washed stone is available, choose it; pea gravel is a compromise, not a spec.
How many inches of gravel go under the pipe?
2-3 inches of bedding stone under the pipe invert, so water can enter perforations from below and the pipe sits on a stable, self-draining bed rather than soil.
Do I fill the trench with gravel to the surface?
For maximum interception, stone comes to within 3-5 inches of grade, then fabric folds over and topsoil/sod caps it. Where surface water should also enter, stone can come fully to the surface (open trench drain style).