French Drain Gravel Calculator

Enter your trench size and pipe to see how much drainage gravel to order.

How the calculation works

The gravel fills the trench below any soil cap, minus the space taken by the pipe:

gravel (cu ft) = length × (width ÷ 12) × (gravel depth ÷ 12) − length × π × (pipe diameter ÷ 24)²

Divide by 27 to get cubic yards. Washed 3/4" drainage stone (often sold as #57 stone) weighs roughly 1.3 to 1.5 tons per cubic yard, so the tonnage is shown as a range. Ask your supplier for their exact figure.

Worked example

A 50 foot trench, 12" wide and 18" deep, with a 4" pipe and gravel to the surface: 50 × 1 × 1.5 = 75 cu ft of trench, minus 4.4 cu ft for the pipe, leaves about 70.6 cu ft. That's 2.6 cubic yards, or about 3.4 to 3.9 tons of stone.

Tips

• Wrap the trench in landscape fabric before adding gravel, so soil doesn't wash in and clog the stone.
• Slope the trench at least 1% (about 1" of drop every 8 to 10 feet) toward the outlet.
• Call 811 before you dig. It's free, and utility lines are marked within a few days.

Frequently asked questions

What size gravel is best for a French drain?

Washed, angular stone between 3/4" and 1-1/2" works best. Round pea gravel drains well too, but it shifts more. Avoid crushed stone with fines, which packs down and slows water.

How much does a cubic yard of gravel cover?

A cubic yard fills about 18 feet of a 12" wide, 18" deep trench, or about 36 feet of a 6" wide trench of the same depth.