French Drain Calculator

Trench volume (length × width × depth) minus the cylinder of the perforated pipe gives the actual gravel needed. Results come in cubic yards with a waste factor and in tons at the editable 1.4 t/yd³ crushed-stone density.

Common uses

• Order drain rock for a yard drainage trench
• Estimate hauling weight for a curtain drain along a foundation

Example

50 ft trench, 12 × 18 in, 4" pipe, 10% waste

75 cu ft trench − 4.4 cu ft pipe → 2.88 yd³ ≈ 4.03 tons

Limitations

• Assumes gravel to the full trench depth; subtract if topping with soil and sod
• Fabric wrap and pipe fittings are not included

How much gravel do I need for a french drain?

Take the trench volume and subtract the pipe. A 50 ft trench 12 in wide and 18 in deep is 75 cubic feet; a 4 in perforated pipe displaces about 4.4 of that, leaving 2.62 cubic yards — roughly 2.88 yd³ with a 10% waste factor, or about 4 tons.

What kind of gravel goes in a french drain?

Clean, washed, angular crushed stone in the 1/2 to 1 in range — commonly sold as drain rock or #57 stone. The washing matters: fines and rounded pea gravel pack together and choke off the void space the drain depends on.

How deep should a french drain be?

18–24 in deep and about 12 in wide handles most yard drainage, which is what this calculator defaults to; a drain protecting a foundation goes deeper so the pipe sits below the slab or footing. Whatever the depth, slope the trench at least 1% — about 1 in of fall every 8 ft — toward the outlet.

Should the holes in the drain pipe face up or down?

Down. Water enters from the gravel bed surrounding the pipe, and holes at the bottom let the pipe start draining as soon as water reaches that level rather than waiting for it to rise several inches higher.

Does this calculator upload my trench measurements?

No. The volume, waste, and tonnage math is done in your browser — the trench dimensions and density you enter are never transmitted or saved.