French Drain Calculator

How much gravel do I need for a french drain?

ft
in
in
Pipe diameter

Order this much drain stone

2.5cubic yards

Stone is washed, with no fines, and includes 10% extra. Lay the pipe to a positive outlet with at least 1% fall. Call 811 before digging.

40 ft long12 in wide18 in deep4 in perforated pipeFabric wrap, dashed2.5 cu yd stoneFall 4.8 in at 1%
Stone weight3.5 tons
Pipe in 10 ft lengths4
Fabric width to wrap6 ft × 40 ft
Fall at 1%4.8 in

A french drain is a stone-filled trench with a perforated pipe, wrapped in fabric. Enter the trench to see stone, pipe and fabric.

How it works

Trench cross-section minus the pipe, times length, gives stone volume with 10% extra. Fabric width covers both sides, the bottom and the top with a 12 inch overlap.

Before you order

  • Use clean, washed stone. Fines will clog the pipe and fabric within a few years.
  • Check for a steady fall to a real outlet with a level and string line.
  • Call 811 before digging, and stay clear of foundations unless a pro has reviewed it.

Common questions

How deep should a french drain be?

Commonly 18 to 24 inches for yard drainage, with a trench about 12 inches wide.

What slope does a french drain need?

At least 1%, about 1 inch of fall per 8 feet. Many installers aim for 1/4 inch per foot.