How Much Gravel for a French Drain?
Quick answer: about 2.5–3.5 tons for a typical 40-foot French drain. A 12-inch-wide, 18-inch-deep trench holds about 0.83 cu ft per linear foot — 40 feet ≈ 33 cu ft (1.23 yards), roughly 2–3 tons of stone.
- Trench dimensions
- Volume per linear foot
- Gravel type matters
- Stone by drain length
- Installation layers
- Worked example: 40-foot drain
- FAQ
Trench dimensions
A French drain trench is typically 12–18 inches wide and 18–24 inches deep. The pipe sits on a 2–3 inch gravel bed, gets 4–6 inches of gravel over the top, and the trench is capped with soil. Deeper and wider trenches move more water but use proportionally more stone.
Volume per linear foot
A 12-inch-wide, 18-inch-deep trench is about 0.83 cu ft per linear foot after allowing for the pipe (12 in × 18 in = 1.5 sq ft, minus the pipe area). Multiply by the length, then divide by 27 for yards or multiply by ~1.4 for tons.
| Drain length | Volume (12×18 in trench) | Tons @ 1.4/yard |
|---|---|---|
| 20 ft | 16.6 cu ft (0.61 yd) | ~0.9 |
| 40 ft | 33.2 cu ft (1.23 yd) | ~1.7 |
| 60 ft | 49.8 cu ft (1.84 yd) | ~2.6 |
| 100 ft | 83 cu ft (3.07 yd) | ~4.3 |
Gravel type matters
Use clean washed gravel or crushed stone in the 1/2–1 inch range — no fines, because dust and sand clog the perforated pipe over time. Washed river rock and 3/4-inch crushed stone are the standard picks. The cleanliness of the stone matters more than the exact size.
Stone by drain length
Order by the ton, and add 5–10% for settling and the loose fill around the top. A 40-foot drain needs about 1.7 tons of stone in the trench; with cover and waste, budget 2–3 tons delivered.
Worked example: a 40-foot drain
- Trench = 12 in wide × 18 in deep = 0.83 cu ft/ft
- 40 ft × 0.83 = 33 cu ft
- 33 ÷ 27 = 1.23 cubic yards
- × 1.4 tons/yard ≈ 1.7 tons → order 2–3
FAQ
About 2.5–3.5 tons for a typical 40-foot French drain. A 12-inch-wide, 18-inch-deep trench is 0.83 cu ft per linear foot; 40 feet is 33 cu ft (1.23 yards), roughly 2–3 tons delivered.
Typically 12–18 inches wide and 18–24 inches deep. The pipe sits on a 2–3 inch gravel bed, is covered with 4–6 inches of gravel, and the trench is capped with soil.
About 0.7–1.0 cu ft per linear foot for a 12-inch-wide, 18-inch-deep trench. Multiply by length, divide by 27 for yards, or use ~1.4 tons/yard.
Clean washed gravel or crushed stone in the 1/2–1 inch range — no fines. Washed river rock and 3/4-inch crushed stone are the standard picks.
Clean pea gravel works if it's washed and free of fines, but 1/2–1-inch crushed stone drains faster. The key is cleanliness — fines clog the pipe.