How Much Gravel for a French Drain? Tons per 100 Feet
Work out the trench volume, take away the pipe, and convert to tons. A 12 inch wide, 18 inch deep French drain with a 4 inch pipe takes about 5.7 cubic yards of gravel per 100 ft, roughly 7.9 tons, with 10% extra. The gravel calculator converts any trench to yards and tons.
Gravel by trench size
Washed stone at about 1.4 tons per cubic yard, with a 4 inch perforated pipe and 10% extra.
| Trench (width × depth) | Cu yd per 100 ft | Tons per 100 ft | Tons per 10 ft |
|---|---|---|---|
| 6 × 12 in | 1.59 | 2.2 | 0.22 |
| 8 × 18 in | 3.62 | 5.1 | 0.51 |
| 12 × 18 in | 5.66 | 7.9 | 0.79 |
| 12 × 24 in | 7.70 | 10.8 | 1.08 |
For any length, multiply the per-10 ft figure. If you're topping the trench with 2 to 4 inches of soil and sod instead of stone, take that depth off before you calculate.
Worked example: 60 ft drain, 12 × 18 inch trench
- Trench cross section: 1 ft × 1.5 ft = 1.5 sq ft
- Minus the pipe: 1.5 − 0.11 = 1.39 sq ft
- Volume: 1.39 × 60 ft × 1.1 = 91.7 cu ft = 3.40 cu yd
- Weight: 3.40 × 1.4 = 4.8 tons
The rest of the shopping list
- Perforated pipe: the trench length plus a few feet to the outlet. 4 inch is standard; lay it holes down.
- Landscape fabric: line the trench and wrap it over the top of the gravel. You need a strip about 72 inches wide for a 12 × 18 inch trench (both sides, bottom, top, plus overlap).
- An outlet: a pop-up emitter, dry well or daylight point lower than the start of the drain.
- Bulk vs bags: 7.9 tons is roughly 318 bags of 50 lb, so for anything over 20 or 30 ft, bulk delivery is far cheaper.
Call 811 before you dig so buried lines are marked.
Frequently asked questions
How much gravel for a 50 ft French drain?
For a 12 × 18 inch trench with a 4 inch pipe, about 2.8 cubic yards, or 4.0 tons, with 10% extra.
How much gravel per foot of French drain?
About 159 lb per foot for a 12 × 18 inch trench, and 44 lb per foot for a narrow 6 × 12 inch trench.
What size gravel is best for a French drain?
Washed, clean 3/4 to 1 1/2 inch stone, such as #57 stone. Avoid crushed stone with fines and pea gravel, which pack down and slow the water.
How deep should a French drain be?
Usually 18 to 24 inches deep and 6 to 12 inches wide, sloping at least 1 inch for every 8 to 10 ft toward the outlet.