A 12-inch wide by 18-inch deep trench needs about 0.056 cubic yards of gravel per linear foot, roughly 0.078 tons of #57 stone. A 50-foot run therefore needs about 2.8 cubic yards, or 3.9 tons, before subtracting pipe volume and any sod cap.

Most yard French drains are 12 to 24 inches deep, with 18 inches the typical standard. Foundation footing drains run 24 to 36 inches or deeper. On flat ground the trench must deepen toward the outlet at least 1/8 inch per foot to create gravity flow.

A standard French drain trench is 9 to 12 inches wide. Twelve inches allows a 4-inch pipe with at least 3 to 4 inches of washed stone on each side, which improves water intake and lets the geotextile fabric wrap fully with a 12-inch overlap on top.

Washed #57 crushed stone, 3/4 to 1 inch angular rock, is the best French drain aggregate. Its roughly 40% void space stores and moves water quickly. Avoid pea gravel, which rolls and shifts, and crusher run, whose fines clog the fabric and pipe.

Perforation holes should face down, positioned at the 4 and 8 o'clock positions. Water rises into the pipe from the saturated stone below, and down-facing holes keep the bottom of the pipe from holding standing water while still letting the line drain completely between storms.

Divide total tons by 0.025, since one 50-pound bag equals 0.025 tons. A 50-foot, 12 by 18 inch trench needs roughly 110 bags with a 4-inch sod cap, or about 155 bags filled to the surface. For anything over a couple of tons, bulk delivery from a quarry is far cheaper than bagged stone.