French Drain Calculator
Enter your trench length, width, and depth to estimate the gravel volume and pipe length needed.
How French drain materials are estimated
This calculator converts your trench width and depth from inches to feet, then multiplies by the trench length to find the total volume in cubic feet, converting to cubic yards (the unit gravel is commonly sold in) by dividing by 27.
The perforated pipe running through a French drain typically matches the trench's full length, since it needs to span the entire drainage path to collect and redirect water along the trench's route.
Frequently asked questions
What size gravel should I use for a French drain?
Clean, washed gravel in the 3/4 inch to 1.5 inch range is common, since smaller gravel can clog more easily and larger gravel leaves oversized gaps. Always wrap the pipe and gravel in landscape fabric to prevent soil infiltration.
Why is gravel measured in cubic yards instead of cubic feet?
Bulk gravel suppliers typically sell and price by the cubic yard, since it's a more practical unit for larger landscaping quantities — a cubic foot would be an inconveniently small unit for most drainage projects.
Does the trench need to be a consistent depth?
No — a French drain should have a slight, consistent downward slope (commonly 1% or about 1/8 inch per foot) toward the drainage outlet so water flows by gravity, rather than a perfectly level trench.