A French drain needs washed #57 crushed stone, about 1.4 to 1.5 tons per cubic yard of trench, after subtracting the pipe. For the common residential trench (12 inches wide, 18 inches deep, 4 inch perforated pipe), that works out to roughly 0.08 tons per linear foot: about 4 tons for a 50 foot run. Use clean stone only; crusher run and its fines will clog the drain.
What a French drain actually is
A French drain is a gravel-filled trench with a perforated pipe at the bottom, sloped so collected water has somewhere to go. Water seeps in through the stone, enters the pipe through its slots, and flows away to a daylit outlet, a dry well or a storm drain. The stone does two jobs: it keeps the trench open as a water channel, and it filters out soil particles (with help from the fabric below) so the pipe slots do not silt up.
It is the standard fix for soggy lawns, water standing at a foundation, and downslope seep. It is not the same as a surface channel drain, which catches sheet flow at a driveway edge; those are shallower and can often use the same #57 stone.
Which gravel to use (this part matters most)
- Best: washed #57 crushed stone. 3/4 to 1 inch, clean, angular, no dust. Water moves through the voids fast, and the pieces are too big to enter the pipe slots. This is the stone nearly every drain is built with.
- Also fine: #5 or small river rock (3/4 to 1.5 in). Rounded and smooth, it drains well and is easier on fabric; slightly less interlocked, which is irrelevant in a drain.
- Never: crusher run, crush and run, #21A/B or any blend with fines. Compaction and dust are virtues in a driveway base and fatal in a drain; see what crusher run is for.
- Not on top of the pipe: pea gravel as the main fill. Small enough to migrate into slots over time. Pea gravel is fine as a decorative surface topper over fabric.
The sizes and types chart shows where #57 sits in the grade system; washed means it has been rinsed of dust at the plant, so specify "washed clean #57" when ordering.
How much gravel: per-foot table
Gravel volume is trench volume minus the pipe, and pipe displacement is worth subtracting (a 4 inch pipe displaces about 6 percent of a 12 inch trench). Stone weight runs about 1.4 to 1.5 tons per cubic yard for #57.
| Trench (width × depth) | Stone per 10 ft | Extends to |
|---|---|---|
| 8 in × 12 in | ~0.4 tons (0.27 yd³) | Shallow lawn strips |
| 12 in × 18 in | ~0.8 tons (0.55 yd³) | Standard residential |
| 12 in × 24 in | ~1.1 tons (0.77 yd³) | Wet sites, curtain drains |
| 18 in × 24 in | ~1.6 tons (1.15 yd³) | Heavy water, downspout feeds |
Rule of thumb from the standard row: about 0.08 tons (160 lb) of stone per foot of trench. A 50 foot run is about 4 tons, a 100 foot run about 8 tons, before the usual 10 percent spare. Order the fabric in the same call: nonwoven geotextile, wide enough to wrap the stone with overlap.
Building one: the short version
- Lay out and dig the trench with a slight fall (2 to 3 inches per 100 feet) toward the outlet.
- Line it with nonwoven landscape fabric, leaving flaps wide enough to wrap over the finished stone.
- Bed of stone: 2 to 3 inches of #57, raked level along the fall.
- Perforated pipe (slots or holes down, depending on the pipe type: follow the manufacturer) on the bed, connected and sloped to the outlet.
- Cover the pipe with stone to within a few inches of grade, then fold the fabric over the top like an envelope.
- Finish: cap with soil and sod for a hidden drain, or a decorative stone topper if you want it visible.
Utilities come first: call the 811 locate service before digging, every time. And if the drain protects a foundation or fights year-round water, have a drainage contractor check the sizing; undersized drains are the most common failure we hear about.
Sizing the order
Measure the trench as a rectangle (length × width × depth in feet), subtract about 0.09 cubic feet per foot for a 4 inch pipe, divide by 27 for yards, multiply by 1.4 for tons, add 10 percent. Or skip the arithmetic: the gravel calculator takes the trench as one area and the calculations are identical. Then price it via washed stone prices (typically $25–$45 per ton bulk), and check why this is always a bulk order: even the small 8 × 12 trench is more stone than you want in bags.