French Drain Material Calculator: Gravel, Pipe and Fall

A French drain is mostly stone, and stone is sold by volume. This calculator turns your trench length, width and depth into cubic yards of gravel, allowing for the space the pipe occupies, and tells you the total fall you need from one end of the run to the other.

When this tool helps most

Use it before ordering materials, and before digging, so you know whether the fall you need is achievable across the route you have in mind.

How to use this tool

1. Measure the trench length along the route you intend to dig.
2. Decide the trench width and the depth of gravel above and below the pipe.
3. Choose the pipe diameter, usually 4 inches for a residential run.
4. Set the fall as a percentage grade; about 1 percent is a common target.
5. Add an allowance for over-dig and settling.

Worked example

A 40 foot trench, 12 inches wide with 18 inches of gravel, a 4 inch pipe and 1 percent fall, needs about 2.30 cubic yards of gravel, roughly 3.2 tons, with 40 feet of perforated pipe and about 180 square feet of filter fabric. The total fall across that run is 4.8 inches.

Understanding your results

Order gravel by volume rather than weight, and confirm the supplier conversion. The fall figure is the one that decides whether the drain works at all: water will not run uphill, and a drain with no fall is a trench that holds water.

Common mistakes to avoid

• Using rounded pea gravel or stone with fines, which clogs instead of draining.
• Digging a trench with no measurable fall along its length.
• Wrapping the pipe in fabric instead of lining the trench, which blinds the pipe faster.
• Digging before calling 811 to have utilities located.

How to get the most accurate result

• Measure your own basement or crawl space with a tape measure instead of using typical sizes.
• Use the numbers printed on your product labels, manuals and data sheets.
• Change one input at a time to see which assumption matters most.
• Print or save the result and bring it to your installer or contractor.

Next steps

Check the ground surface too with the Yard Drainage Grading Calculator, since surface water is usually the larger source.

Frequently asked questions

How much fall does a French drain need?

A commonly used target is about 1 percent, roughly 1 inch of drop in 8 feet. More is better where the route allows it. The calculator shows the total drop across your run so you can check it is achievable.

What stone should I use?

Washed, angular stone without fines. The voids between the stones are the drainage path, so anything that fills them defeats the purpose.

Does the fabric go round the pipe?

Line the trench with it rather than sleeving the pipe. A sock on the pipe tends to blind off with silt sooner.

This tool is for planning and education only. It does not store your entries. Structural judgement belongs to a licensed structural engineer, radon mitigation design to a certified mitigator, and mold over roughly 10 square feet to a remediation professional. New circuits are a job for a licensed electrician. Read our disclaimer.