French Drain Gravel Calculator

Getting the stone right

A French drain works because water moves freely through the void space between stones. That means the stone must be washed — free of the fine dust left over from crushing. Washed #57 stone, roughly three quarters of an inch, is the standard and the right answer for almost every residential drain.

Using crusher run or any stone with fines in it is the classic mistake. The fines migrate into the voids, silt binds them together, and within a few years the drain is a buried trench of damp aggregate that moves no water at all. It is not repairable — it has to be dug out and rebuilt.

Why the pipe volume is subtracted

This calculator subtracts the volume the perforated pipe occupies from the stone quantity, which many simpler calculators do not. On a four-inch pipe it is a small correction, but across a long drain it adds up to a meaningful fraction of a cubic yard, and over-ordering washed stone is not cheap.

Fall is non-negotiable

A French drain must slope continuously toward its outlet at a minimum of 1% — one inch of drop for every eight feet of run. A drain with a flat section or a dip will hold water at that point, silt up there first, and eventually block. Set a string line at the target grade before you dig and check the trench bottom against it as you go, rather than trusting the ground to fall evenly.

Wrapping the trench

Geotextile fabric goes around the whole stone envelope, not just underneath it. Lay it into the open trench with generous overlap up both sides, place the pipe and stone, then fold the excess over the top before capping. This is what keeps soil fines out of the drainage voids from every direction, and it is the difference between a drain that lasts twenty years and one that lasts five.

Sock-wrapped pipe is a useful addition but is not a substitute for wrapping the trench — the sock protects the pipe perforations, while the fabric protects the entire stone bed.

Where the water goes

A French drain moves water; it does not make it disappear. Before you dig, decide where the outlet discharges — daylight on a slope, a dry well, or a storm connection where local code permits it. Draining water from one part of your property to another part that cannot take it is a common and frustrating outcome.