How Much Gravel Do You Need for a French Drain?

Updated 2026-09-12 · Yardly

A 100-foot French drain in a common 12-inch-wide, 18-inch-deep trench needs about 6 cubic yards of washed 3/4-inch gravel — the stone fills the whole trench around a 4-inch perforated pipe, so you size the trench box, not a thin layer. Here’s the formula, the slope and pipe cover that change the number, and the fabric step most people skip.

The French drain gravel formula

A French drain is a gravel-filled trench, so the volume is the trench box minus a sliver for the pipe:

(Trench length × Width × Depth ÷ 12) ÷ 27 × 1.10 = cubic yards of gravel

Length and width in feet, depth in inches (the ”÷ 12” turns it into feet), ”÷ 27” is cubic feet to cubic yards, and ”× 1.10” is a waste and cap margin — you lose some stone to spillage and leave a few inches of cap above the pipe. The 4-inch pipe void is tiny next to the trench, so ignore it.

The gravel calculator runs this from your trench dimensions and switches between yards and tons.

Worked examples

100 ft drain, 12 in wide, 18 in deep (common DIY):

• 100 × 1.0 × 1.5 = 150 cu ft ÷ 27 = 5.6 cu yd
• × 1.10 = 6.1 cu yd → order ~6½; × 1.4 tons ≈ 9 tons

100 ft drain, 8 in wide, 18 in deep (tight space):

• 100 × 0.667 × 1.5 = 100 cu ft ÷ 27 = 3.7 cu yd
• × 1.10 = 4.1 cu yd → order ~4½; × 1.4 tons ≈ 6 tons

100 ft drain, 12 in wide, 24 in deep (full-size, closer to a pro job):

• 100 × 1.0 × 2.0 = 200 cu ft ÷ 27 = 7.4 cu yd
• × 1.10 = 8.1 cu yd → order ~8½; × 1.4 tons ≈ 11 tons

A wider, deeper trench is the same math with a bigger box — size the trench to the water problem, not to a maximum, because the stone volume climbs fast.

The stone: clean and angular

This is not decorative gravel. A French drain needs void space — channels for water to fall through the stone and reach the pipe. That means:

• Washed #57 or #2 crushed stone (3/4-inch angular). The gradation classes are standardized in ASTM D448, so the designation means the same thing from one quarry to the next.
• Never pea gravel or river rock. Rounded stone packs into a near-solid mass with no drainage voids, and it shifts under load.
• Skip decomposed granite and fines. They wash into the pipe and clog it.

The Interlocking Concrete Pavement Institute takes the same position on angular aggregate for any load- or water-bearing section: interlock beats round every time.

Slope, pipe cover, and fabric

Three details decide whether the drain works past year two:

1. Slope ~1%. Fall the trench about 1/8 inch per foot toward the outlet. A flat drain just holds water.
2. 6–12 inches of stone over the pipe. The pipe sits on a fabric-wrapped stone bed with stone above it; the cap keeps surface loads off the pipe and gives fines a place to settle before they reach it.
3. Geotextile fabric on the trench walls and bed. Non-woven fabric stops soil fines from washing in and sealing the voids — the single most common French drain failure is a silted-up trench with no fabric.

Keep reading

• Sizing the raw trench volume? The gravel calculator turns length, width, and depth into yards and tons.
• Same stone under a shed instead? The gravel shed base guide covers the thinner, oversized pad section.
• Want the ton math before you call the yard? Cubic yards to tons lists the density factor for washed stone and everything else.

Frequently asked questions