How Much Gravel for a French Drain? Tons and Yards per Foot Chart
A 50 ft French drain in a trench 12 inches wide and 18 inches deep, with a 4 inch pipe, takes about 2.9 cubic yards (4.0 tons) of drain rock, including 10% extra for uneven trench walls. Use the French drain gravel calculator for your own trench, or read your size from the charts below.
Part of the project planner: Planning a paver patio or garden path.
Gravel by trench size and length
Drain rock with a 4 inch pipe and 10% extra, in cubic yards and tons (t), at 1.4 tons per cubic yard:
| Trench (width × depth) | Stone per foot | 10 ft drain | 25 ft drain | 50 ft drain | 100 ft drain |
|---|---|---|---|---|---|
| 8 × 12 in | 0.64 ft³ | 0.2 yd³ (0.3 t) | 0.6 yd³ (0.8 t) | 1.2 yd³ (1.7 t) | 2.4 yd³ (3.3 t) |
| 8 × 18 in | 1.00 ft³ | 0.4 yd³ (0.5 t) | 0.9 yd³ (1.3 t) | 1.9 yd³ (2.6 t) | 3.7 yd³ (5.2 t) |
| 12 × 12 in | 1.00 ft³ | 0.4 yd³ (0.5 t) | 0.9 yd³ (1.3 t) | 1.9 yd³ (2.6 t) | 3.7 yd³ (5.2 t) |
| 12 × 18 in | 1.55 ft³ | 0.6 yd³ (0.8 t) | 1.4 yd³ (2.0 t) | 2.9 yd³ (4.0 t) | 5.8 yd³ (8.1 t) |
| 12 × 24 in | 2.10 ft³ | 0.8 yd³ (1.1 t) | 1.9 yd³ (2.7 t) | 3.9 yd³ (5.5 t) | 7.8 yd³ (10.9 t) |
| 18 × 24 in | 3.20 ft³ | 1.2 yd³ (1.7 t) | 3.0 yd³ (4.2 t) | 5.9 yd³ (8.3 t) | 11.9 yd³ (16.6 t) |
The formula
- Trench volume (ft³) = length × (width ÷ 12) × (depth ÷ 12).
- Stone = trench volume less the volume of the pipe, then add the allowance for waste.
- Cubic yards = ft³ ÷ 27, and tons = cubic yards × 1.4 for typical washed stone.
Without the 10% allowance the 50 ft example is 2.62 cubic yards. The pipe takes up only a little of the trench, so the pipe size barely changes the order:
| Pipe | Drain rock | Weight |
|---|---|---|
| 3 in | 2.96 yd³ | 4.1 tons |
| 4 in | 2.88 yd³ | 4.0 tons |
| 6 in | 2.66 yd³ | 3.7 tons |
Ordering the stone
- Bulk stone from a landscape supplier is sold by the ton or the cubic yard. Bags of drainage gravel are usually only worth it for a few feet of drain.
- Ask for clean, washed stone of about 3/4 to 1½ inches. Fines (dust) wash into the pipe and clog it.
- Wrap the stone in non-woven filter fabric so soil cannot wash in.
- Slope the trench toward the outlet: see slope and grade for drainage.
The pipe, fabric, fittings and tools are in the French drain materials list, and the French drain materials calculator totals the whole job. For gravel under a patio or driveway use the gravel calculator.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply length × width × depth, subtract the pipe, and add 10%. A 50 ft drain in a 12 × 18 inch trench with a 4 inch pipe needs about 2.9 cubic yards, or 4.0 tons, of drain rock.
How many tons of gravel per foot of French drain?
In a 12 × 18 inch trench, about 0.081 tons per foot, or 1.55 cubic feet. For 100 feet that is about 8.1 tons.
What size gravel is best for a French drain?
Clean, washed, angular stone of about 3/4 to 1½ inches, with no fines. Do not use crusher run, stone dust or sand, which clog the drain.
How much does a cubic yard of drain rock weigh?
About 1.4 tons, the figure the calculator uses. Ask your supplier for the weight of their stone, since it varies with the rock type.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.
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