A french drain is a gravel-filled trench, so the gravel is most of what you buy: multiply the trench width by depth by length in feet, subtract the pipe, and divide by 27 for cubic yards. A 40 foot trench 12 inches wide and 18 inches deep with a 4 inch pipe takes about 2.1 cubic yards, roughly 2.9 tons of washed #57 stone.
The amount is easy. What actually decides whether the drain works is the kind of gravel and the slope, and those are where most french drains fail. Here is all three.
How much gravel, by trench size
Each row is a trench 12 inches wide and 18 inches deep with a 4 inch perforated pipe, in washed #57 stone at 1.4 tons per cubic yard:
| Trench length | Gravel | Weight |
|---|---|---|
| 20 ft | 1.05 cu yd | 1.5 tons |
| 40 ft | 2.09 cu yd | 2.9 tons |
| 60 ft | 3.14 cu yd | 4.4 tons |
| 100 ft | 5.23 cu yd | 7.3 tons |
A 4 inch pipe only displaces about 0.09 cubic feet per foot of run, so it trims a hair under 3.5 cubic feet from a 40 foot trench: real, but small. The french drain calculator subtracts it for you and also sizes the filter fabric and the soil you will dig out, which is the same volume as the full trench before the gravel goes back in.
Order a little extra. Hand-dug trenches never come out to exact dimensions, and running short in the middle of spreading stone means a second delivery.
The gravel type matters more than the amount
This is the decision that makes or breaks the drain. Use clean, washed, angular crushed stone, usually 3/4 inch #57. Its large open voids carry water fast, and because it has been washed there are no fines to pack into the gaps.
Do not use anything with stone dust or fines in it: crusher run, dense-grade, #411, or a cheap pea-gravel blend. Those materials are made to compact into a hard surface, which is the exact opposite of what a drain needs. The fines migrate into the voids, water stops moving, and the drain silts solid within a season or two. If a supplier offers washed and unwashed at a small price difference, pay for washed every time.
Wrap it in filter fabric
Soil washing into the stone voids is the other common way a drain dies, and non-woven filter fabric is the cheap insurance against it. Line the open trench with fabric, fill it with gravel and pipe, then fold the excess over the top like a burrito before capping with soil or sod.
Size the fabric for the wrap: its width needs to cover the trench bottom, up both sides, and overlap on top, so figure the trench width plus twice the depth plus about a foot of overlap. You can skip fabric only in clean, sandy ground that will not migrate; in silt or clay it is the difference between a drain that lasts decades and one that fails in a few years. A pipe sock adds a second layer in the dirtiest soil.
Slope and the outlet decide the rest
Gravel and fabric are nothing without fall. The trench bottom has to drop continuously toward the outlet, at least 1 percent, about 1 inch for every 10 feet of run, with no low spots where water can pool. Check it with a string line and a level before any stone goes in, because once the gravel is down a flat spot is invisible and permanent.
Lay the perforated pipe with its holes facing down. It feels backward, but groundwater rises into the pipe from below, so slots facing down let it fill and drain at the lowest point. And the whole system is only as good as where it ends: run it to daylight on a slope, a dry well, or a storm connection. A drain that empties into a low spot with nowhere to go just relocates the puddle.
Tying roof runoff into the same trench? The downspout guide sizes where that water comes from, and the gravel calculator handles any separate gravel you need for the base or the outlet.