How Much Gravel for a 40 Foot French Drain?

A 40 ft French drain in a trench 12 in wide and 18 in deep takes about 2.3 cubic yards (3.2 tons) of drain rock, with a 4 in pipe and 10% extra. You also need 40 ft of perforated pipe and 200 sq ft of filter fabric, and you will dig out 2.2 cubic yards of soil.

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Example job shown. Change the numbers to match yours.

Drain rock needed

3.2 tons

Clean washed stone, 3/4 to 1½ in

Drain rock, cubic yards: 2.30 yd³

Pipe and fabric

Perforated pipe: 40 linear ft
10 ft pipe sections: 4 pieces
Filter fabric (wraps the stone, with overlap): 200 ft²

Digging

Soil to dig out: 2.2 yd³

Lay the pipe with the holes down, on a bed of stone, sloping at least 1 percent (about 1/8 in per foot) to a daylight outlet or a dry well. Wrap the stone in non-woven filter fabric to keep soil out. Soil is bulkier once it is dug, so the pile to haul away is bigger than the soil to dig out: allow extra and ask your hauler.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

Blueprint

2.30 yd³ gravel

What a 40 ft French drain takes

Item	Amount
Drain rock (clean washed stone, 3/4 to 1½ in)	2.30 cubic yards, 3.2 tons
Perforated pipe, 4 in	40 ft (4 pieces of 10 ft)
Non-woven filter fabric	200 sq ft
Soil to dig out	2.2 cubic yards

For a trench 12 in wide and 18 in deep, with 10% extra stone for uneven trench walls and settling. The stone is the trench volume less the space the pipe takes up, at 1.4 tons per cubic yard.

40 ft of drain, by trench size

Width × depth of the trench, with a 4 in pipe and 10% extra stone.

Trench	Drain rock, cubic yards	Tons	Fabric, sq ft	Soil dug out, cubic yards
8 × 12 in	0.94	1.3	147	1.0
12 × 12 in	1.49	2.1	160	1.5
12 × 18 in	2.30	3.2	200	2.2
12 × 24 in	3.12	4.4	240	3.0
18 × 24 in	4.75	6.6	260	4.4

3, 4 or 6 inch pipe?

The pipe takes up space the stone would otherwise fill, so a bigger pipe means slightly less rock. For a 40 ft drain in a 12 by 18 in trench: 2.36 cubic yards with 3 in pipe, 2.30 with 4 in and 2.12 with 6 in.

The difference between 3 in and 6 in is only 0.3 tons, so choose the pipe for the water it has to carry, not for the stone it saves. Four inch is the size the calculator starts on.

Use clean, washed stone with no fines, wrap the stone (not the pipe) in the fabric, and make sure the drain has an outlet. The fittings, tools and steps are in the French drain materials list, and the method behind the stone figure is in how much gravel for a French drain.

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Tools & materials for this project

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Pipe fittings and couplers

Frequently asked questions

How much gravel do I need for a 40 foot French drain?

About 2.3 cubic yards, or 3.2 tons, of clean drain rock for a trench 12 in wide and 18 in deep with a 4 in pipe, including 10% extra. A 12 by 24 in trench takes 3.1 cubic yards.

Does the pipe size change the gravel for a French drain?

Only a little. A 40 ft drain takes 3.3 tons with 3 in pipe, 3.2 with 4 in and 3.0 with 6 in, in a 12 by 18 in trench.

How much pipe for a 40 ft French drain?

40 ft of perforated pipe: 4 pieces of 10 ft, or one coil of at least 40 ft.

Related sizes and guides

Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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