French Drain Gravel Calculator (Subtracts the Pipe)

Trench length Trench width Gravel depth in the trench From the bottom of the trench to the top of the stone: the bed under the pipe, around it and over it. Pipe diameter Enter 0 if there is no pipe. 4 inch pipe is the most common for yard drains. Density from your supplier (optional) Waste (optional)%

Gravel per 10 feet of trench with a 4 inch pipe

Cubic feet of gravel per 10 ft, pipe volume already taken out
Trench width	12 in deep	18 in deep	24 in deep
6 in	4.13 ft³	6.63 ft³	9.13 ft³
9 in	6.63 ft³	10.38 ft³	14.13 ft³
12 in	9.13 ft³	14.13 ft³	19.13 ft³

Divide by 27 for cubic yards. Multiply by the length of your run in tens of feet.

What gravel to use in a French drain

Use washed, angular stone with no fines, roughly 3/4 to 1-1/2 inch. #57 stone is a common choice. Fines and rounded pea gravel pack tighter and leave less room for water to move.

Many people line the trench with filter fabric and fold it over the top of the stone so soil does not wash in and clog it.

How to measure the trench

Measure the full run of the trench in feet. Width is usually given in inches; a spade-width trench is about 6 inches, and 9 to 12 inches is common for a drain with a 4 inch pipe. For the gravel depth, measure from the trench bottom to where the stone will stop, which is usually a few inches below the surface so you can cover it with soil or sod.

Call 811 a few days before you dig so buried lines are marked.

Worked example: a 50 ft drain

Trench: 50 ft long, 12 in (1 ft) wide, gravel 18 in (1.5 ft) deep.
Trench volume: 50 × 1 × 1.5 = 75 ft³.
Pipe, 4 in across: π × (0.167 ft)² × 50 ft = 4.36 ft³.
Gravel: 75 − 4.36 = 70.64 ft³, or 2.62 cubic yards.
That is about 142 bags of 0.5 ft³, so for a run this long a bulk delivery is usually cheaper.

Common questions

Does the pipe really change how much gravel I need?

A little. A 4 inch pipe in a 12 × 18 inch trench takes up about 6 percent of the space. In a narrow 6 × 12 inch trench it is about 17 percent, which is worth knowing when you buy bags.

How much gravel do I need per foot of French drain?

Width × depth in feet gives cubic feet per foot of trench, minus the pipe. A 12 × 18 inch trench with a 4 inch pipe needs about 1.41 cubic feet per foot.

Can I use pea gravel in a French drain?

It works, but angular washed stone is the usual choice because it locks in place and leaves more open space for water.