How much gravel for a French drain? Per foot and per 50 or 100 ft
A French drain in a 12 inch wide trench 18 inches deep takes about 1.55 cubic feet of gravel per foot once the 4 inch pipe is taken out and 10% is added for uneven trench walls; at 24 inches deep it is 2.1 cubic feet per foot. So a 50 ft drain at 18 inches needs 2.9 cubic yards, about 3.6 tons, and a 100 ft drain at 24 inches 7.8 cubic yards, 9.9 tons. Bagged gravel stops making sense after the first 10 feet.
Gravel per foot and per run, trench filled to the top
Twelve inch wide trench, 4 inch perforated pipe taken out, washed gravel at 1.5 tonnes per cubic metre, 10% extra because trench walls are never straight. Filled to the surface: the usual detail beside a driveway or where the gravel top is the finish.
| Length | Cubic feet | Cubic yards | US tons | Bags (0.5 cu ft) |
|---|---|---|---|---|
| 1 ft | 1.6 | 0.06 | 0.07 | 4 |
| 10 ft | 15.5 | 0.58 | 0.73 | 32 |
| 25 ft | 38.9 | 1.44 | 1.82 | 78 |
| 50 ft | 77.7 | 2.88 | 3.64 | 156 |
| 100 ft | 155.4 | 5.76 | 7.28 | 311 |
| Length | Cubic feet | Cubic yards | US tons | Bags (0.5 cu ft) |
|---|---|---|---|---|
| 1 ft | 2.1 | 0.08 | 0.1 | 5 |
| 10 ft | 21 | 0.78 | 0.99 | 43 |
| 25 ft | 52.6 | 1.95 | 2.46 | 106 |
| 50 ft | 105.2 | 3.9 | 4.93 | 211 |
| 100 ft | 210.4 | 7.79 | 9.85 | 421 |
Covered drain: gravel stops 4 inches below the lawn
Under a lawn the gravel usually stops 4 inches short of the surface and the trench is finished with soil and turf over the fabric. Same 24 inch trench, 20 inches of gravel.
| Length | Cubic feet | Cubic yards | US tons |
|---|---|---|---|
| 1 ft | 1.7 | 0.06 | 0.08 |
| 10 ft | 17.4 | 0.64 | 0.81 |
| 25 ft | 43.4 | 1.61 | 2.03 |
| 50 ft | 86.9 | 3.22 | 4.07 |
| 100 ft | 173.7 | 6.43 | 8.13 |
How the numbers are worked out
Trench: 1 ft wide x 1.5 ft deep x length = 1.5 cubic feet per foot at 18 inches. Pipe: pi x (2 in)² x 12 in = 0.087 cubic feet per foot, subtracted. Add 10%, then divide by 27 for yards or multiply by the density for tons. The gravel calculator takes the trench as an area (width x length) and a depth; knock off the pipe by entering the depth minus about an inch, or run it exact with the figures above.
Metric: 300 mm trench, 100 mm pipe
| Length | Depth | Cubic metres | Tonnes | Bulk bags |
|---|---|---|---|---|
| 10 m | 450 mm | 1.4 | 2.1 | 3 |
| 25 m | 450 mm | 3.5 | 5.24 | 7 |
| 50 m | 450 mm | 6.99 | 10.49 | 13 |
| 10 m | 600 mm | 1.89 | 2.84 | 4 |
| 25 m | 600 mm | 4.73 | 7.1 | 9 |
| 50 m | 600 mm | 9.47 | 14.2 | 17 |
Before you dig
- Call the utility locating service; drains cross gas, water and cable lines more often than any other garden job.
- Set the fall with a string line and a level before ordering: a trench that has to be deeper at the outlet end needs the deeper table for that half of the run.
- Fabric first, then 2 to 3 inches of gravel, the pipe with the holes facing down, gravel over, fabric folded across the top. The fabric is what keeps the gravel working for decades.
- Gravel for a driveway or path is worked out the same way in the driveway guide; what a yard of it weighs is in the cubic yard weight guide.
Frequently asked questions
- How much gravel for a 100 ft French drain?
- 5.8 cubic yards (7.3 tons) for a 12 inch trench 18 inches deep filled to the top, 7.8 cubic yards (9.9 tons) at 24 inches. If you cover the top 4 inches with soil and sod, use the covered table: 6.4 yards at 24 inches.
- How much gravel for a 50 ft French drain?
- 2.9 cubic yards or 3.6 tons at 18 inches deep, 3.9 yards at 24 inches, in a 12 inch wide trench with 10% extra. That is 156 bags of 0.5 cubic feet at 18 inches: order it loose.
- How much gravel goes under and around the pipe?
- 2 to 3 inches of gravel under the pipe, then gravel around and over it to within 4 inches of the surface (or all the way up for a gravel-topped drain). The tables assume the whole trench is gravel except the pipe itself; the covered table stops 4 inches short for a soil and turf top.
- What size gravel for a French drain?
- Washed, rounded or crushed stone between 3/4 and 1 1/2 inches, with no fines: the gaps between the stones are what carry the water. Do not use crusher run, road base or pea gravel that migrates into the pipe slots. Wrap the trench in a non-woven geotextile so soil cannot silt the gravel up.
- Does the pipe change the gravel quantity much?
- A 4 inch pipe displaces 0.087 cubic feet per foot, about 6% of an 18 inch trench. It is worth taking out on a long run, and the tables do; on a 10 ft drain it is half a bag.
- How wide and deep should a French drain be?
- 12 inches wide is the usual trench for a 4 inch pipe (wider is more gravel for little gain), 18 to 24 inches deep for surface water and lawn drainage, deeper when it has to reach below a footing. The bottom needs a fall of at least 1% (1 inch per 8 ft) towards the outlet, so the far end is deeper than the start.
More guides
- How much gravel do I need for a driveway?
Cubic metres, tonnes and bulk bags of gravel for typical driveway sizes at 50, 75 and 100 mm depth, with the imperial equivalents in cubic yards and tons.
- How much does a cubic yard of gravel weigh?
A cubic yard of gravel weighs about 1.2 to 1.4 US tons; sand and topsoil differ. Pounds and tons per yard, yards per ton, and tonnes per cubic metre for each material.
- How much gravel for a shed base? 6x8 to 12x16 ft
Cubic yards, tons and bags of gravel for 6x8, 8x10, 10x12 and 12x16 ft shed bases at 4 and 6 inches, why the base runs past the shed, and metric sizes in tonnes.
- How much sand and gravel do I need under pavers?
Gravel base at 4 or 6 inches and 1 inch of bedding sand for 100 to 500 sq ft of pavers: cubic yards, tons and bags, plus the metric patio in tonnes and bulk bags.
Last updated 2026-09-12. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.