French Drain Calculator: Gravel and Pipe You Need
A french drain moves water away from a wet spot or foundation. Here is how to size the trench and the materials.
What a french drain is
A french drain is a gravel-filled trench with a perforated pipe at the bottom, wrapped in landscape fabric. Water in the surrounding soil enters the gravel, drops into the pipe, and flows to a lower discharge point.
Trench dimensions
A typical yard drain is 12 inches wide and 18 to 24 inches deep. Wider or deeper trenches move more water but take more gravel and more labor.
The gravel volume math
Multiply length by width by depth in feet to get cubic feet, then divide by 27 for cubic yards. Inches convert to feet by dividing by 12.
Worked example
A 40 foot trench at 12 inches wide and 18 inches deep is 40 times 1 times 1.5, or 60 cubic feet. Divide by 27 and you get about 2.2 cubic yards of drainage stone.
Pipe length and slope
Perforated pipe runs the full length of the trench. Slope matters: aim for at least 1 percent, or about 1 inch of drop per 10 feet of run, so water actually moves toward the outlet.
Frequently asked questions
How deep should a french drain be?
Most yard drains are 18 to 24 inches deep. Foundation drains go deeper, ideally down to the level of the footing so water is intercepted before it reaches the wall.
What gravel do I use in a french drain?
Clean, washed drainage stone in the 3/4 inch to 1.5 inch range works best. Avoid fines and stone dust, which clog the pipe and defeat the point of the drain.
Do I need landscape fabric?
Yes. Wrapping the trench with a non-woven filter fabric keeps soil fines out of the gravel and pipe, which is the main failure point of drains built without it.
Try the calculator
Get gravel and pipe quantities in seconds with our French Drain Calculator.