French Drain Gravel Calculator

A French drain is a gravel-filled trench with a perforated pipe that redirects groundwater away from problem areas like foundations, yards, and basements. The gravel is the part that matters. It has to be coarse enough for water to move freely through the voids, and clean enough that fine soil cannot wash in and choke it.

French Drain Construction

A standard French drain consists of:

1. A trench, typically 12–24 inches wide and 18–36 inches deep
2. A layer of landscape fabric (geotextile) lining the trench
3. A 3–6 inch bed of gravel at the bottom
4. A perforated pipe (4-inch diameter is most common)
5. Gravel filling the rest of the trench (leaving 2–3 inches for topsoil)
6. Landscape fabric folded over the top
7. Topsoil or sod to finish

Choosing the Right Gravel

• #57 crushed stone (¾ inch): The default, and what most contractors use. Open-graded, so roughly a third of the volume is void space for water to travel through.
• #4 crushed stone (1.5 inch): More void space still, for high-flow situations. Harder to shovel and it will not sit tight around the pipe.
• Pea gravel (⅜ inch): Rounded and easy to pour, but the round shape lets it consolidate over the years and it holds less water than it looks like it should.
• River rock: Fine as a decorative surface course over the top. Heavier per yard and no better hydraulically.

The number, “#57” or “#4”, is a size specification, not a rock type. Ask for washed #57 and you will get whatever your local quarry cuts, usually limestone in the eastern US and granite or basalt further west. Either is fine here.

What Actually Ruins a French Drain

• Crusher fines, screenings or stone dust. Same rock, wrong size. These pack into something close to concrete and the drain stops draining. This is the mistake worth avoiding, and it gets made because the pile at the yard is cheap and looks like gravel.
• Unwashed stone. If it comes coated in quarry dust, that dust ends up in the voids.
• Sand or fine gravel. Clogs within a season or two.
• Decomposed granite. Breaks down into fine particles by design.

The other half of the job is the geotextile. Wrap the stone, not just the pipe, and overlap the fabric generously at the top. Silt migrating in from the surrounding soil is what kills these drains, usually about eight years after whoever built it has moved house.

Volume Calculation

The trench is a rectangular prism: Volume (ft³) = Length (ft) × Width (ft) × Depth (ft) Volume (yd³) = Volume (ft³) ÷ 27

Metric: Volume (m³) = Length (m) × Width (m) × Depth (m)

Gravel Weight

Washed #57 crushed stone weighs roughly 2,700 lbs per cubic yard, which is 1,600 kg per m³. The Gravel Weight Calculator carries the full density table if you are ordering a different material.

The pipe takes up room too

A 4-inch perforated pipe is about 4.5 inches across the outside, so it displaces roughly 0.11 cubic feet for every foot of trench. Against a 12-inch wide by 18-inch deep trench that is about 7% of the gravel, and against a wide 18 by 24 inch trench closer to 4%. This calculator works it out from your own trench size and subtracts it, so the figure you are given is stone, not stone plus pipe.