Plan on about 0.56 cubic yards of stone per 10 feet of french drain — call it 3 yards for a 50-foot run. That assumes the classic trench: 12 inches wide, 18 inches deep, pipe bedded in stone with fabric wrapping the works. Keep 2–3 inches of stone under and beside the pipe and 6 or more above it; the stone envelope, not the pipe, does most of the collecting.
Stone per trench length
| Trench length | ¾" stone needed |
|---|---|
| 10 ft | ~0.6 cy |
| 25 ft | ~1.5 cy |
| 50 ft | ~3 cy |
Angular Gray Granite ¾" holds open voids so water moves to the pipe instead of through your lawn. Adjust for your trench size with the material calculator — and subtract a hair for the pipe itself.
