How Much Gravel for a French Drain? Yards and Tons for 10 to 100 Feet

A 12-inch-wide, 18-inch-deep French drain takes 1.5 cubic feet of gravel per foot of trench. That is about 2.8 cubic yards, or roughly 4.2 tons, for a 50-foot drain. The Northern Virginia Soil and Water Conservation District puts a standard French drain trench at about 1.5 feet deep and 10 to 12 inches wide. Tons use Penn State's average of 1.5 tons per cubic yard for #57 stone.

Free calculator: our free gravel calculator turns trench length, width and depth into cubic yards, tons and bags. No sign-up.

How much gravel per foot of French drain

Figures fill the whole trench with gravel, as you would if the gravel stays exposed at the surface. Tons use 1.5 tons (3,000 lb) per cubic yard.

12 × 18 in trench, 10 ft — 15 cu ft; 0.56 cu yd; 0.83 tons

12 × 18 in trench, 25 ft — 37.5 cu ft; 1.39 cu yd; 2.08 tons

12 × 18 in trench, 50 ft — 75 cu ft; 2.78 cu yd; 4.17 tons

12 × 18 in trench, 100 ft — 150 cu ft; 5.56 cu yd; 8.33 tons

10 × 18 in trench, 50 ft — 62.5 cu ft; 2.31 cu yd; 3.47 tons

10 × 18 in trench, 100 ft — 125 cu ft; 4.63 cu yd; 6.94 tons

The pipe takes back a little room. A 4-inch pipe fills about 0.09 cu ft per foot, so these figures leave about 6% to 8% to spare in a 12 × 18 trench.

Covering the drain with topsoil and sod? Count gravel only to the depth it will reach. For example, leaving the top 4 inches for soil gives 14 inches of gravel in a 12 × 18 trench: 1.17 cu ft per foot, or 2.16 cu yd for 50 ft.

Bags add up fast. A 10-foot drain takes 30 half-cubic-foot bags, and a 50-foot drain takes 150. For longer runs, price bulk gravel by the ton.

How to build a French drain

Dig the trench. About 1.5 ft deep and 10 to 12 in wide. The Northern Virginia district says the size depends on the pipe, usually 4 to 6 inches, and on how deep you want the drain.

Give it slope. Washington State University Extension suggests a slope of 0.5% to 1%. One foot of drop over 100 ft is 1%, so a 50-foot drain needs 3 to 6 inches of fall from end to end.

Line it with fabric. Line the trench on all sides with landscape fabric. WSU Extension says the fabric keeps soil from filling the spaces between the gravel, so water keeps moving.

Gravel, pipe, gravel. Put a layer of gravel in the bottom, lay perforated pipe on it, then surround the pipe with gravel on the sides and top.

Close it up. If topsoil and sod go on top, pull the fabric over the gravel first. If the gravel stays exposed, the fabric can stay open, but protect the edges so sediment cannot wash in.

Send the water somewhere safe. The pipe has to end at a suitable outlet. The Northern Virginia district warns that dumping runoff in the wrong place just moves the problem downhill and can affect your neighbors. WSU notes that a T-shaped pipe at the outlet slows the water and spreads it out.

For surface runoff, WSU also says French drains work best above the groundwater table, and that heavy machinery should stay off them because it can crush the pipe.

How much fabric to buy

To line a 12 × 18 in trench and fold the fabric closed over the top, you need 5 ft of width: 12 in for the bottom, 18 in for each side and 12 in across the top. A 6-ft-wide roll leaves about a foot of overlap. If the gravel stays exposed, 4 ft of width covers the bottom and sides.

Fabric and a cart for a French drain (#ad)

As an Amazon Associate, Curb Calc earns from qualifying purchases.

Super Geotextile 4 oz Non Woven Fabric, 6 × 50 ft — a nonwoven geotextile that the maker lists for French drains and calls UV and rot resistant. The 6-ft width wraps a 12 × 18 in trench with about a foot to spare, and one 50-ft roll just covers a 50-foot drain, so buy extra for the outlet or a longer run. See it on Amazon

BILT HARD 900 lb Poly Garden Dump Cart — 10-inch pneumatic tires, a quick-release dump and a 900 lb rating, with a 400 lb maximum when dumping, per the maker. At 400 lb a load, the 4.17 tons for a 50-foot drain is about 21 trips. See it on Amazon

French drain gravel FAQ

How much gravel do I need for a 100 ft French drain? For a 12 × 18 in trench filled with gravel, 150 cu ft: 5.56 cubic yards, or about 8.3 tons at 1.5 tons per cubic yard.

How much gravel for a 50 ft French drain? 75 cu ft for a 12 × 18 in trench. That is 2.78 cubic yards, or about 4.2 tons.

How deep and wide should a French drain be? About 1.5 ft deep and 10 to 12 in wide, per the Northern Virginia Soil and Water Conservation District. The size depends on the pipe, usually 4 to 6 inches.

How much slope does a French drain need? Washington State University Extension suggests 0.5% to 1%, which is 6 to 12 inches of drop per 100 ft.

Do I need fabric for a French drain? Both drainage sources line the trench with fabric. WSU Extension says it keeps soil out of the spaces between the gravel so water keeps flowing.

Free calculator: enter your trench length, width and depth in the free gravel calculator. Set the weight to 3,000 lb per cubic yard to match this chart, or use your supplier's number.

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Sources: Northern Virginia Soil and Water Conservation District, Control Heavy Runoff; Washington State University Extension, Improving Drainage; Penn State Center for Dirt and Gravel Road Studies, DGLVR Materials Calculator.