How a French drain actually works
A French drain is a gravel filled trench that gives groundwater an easier path than your soil, with a perforated pipe at the bottom carrying the collected water away. Water always takes the path of least resistance; the drain wins because clean stone has 35 to 40 percent void space against a few percent in compacted soil. That void space is also why the material choice is non negotiable: washed #57 stone drains, crusher run seals.
The math this calculator runs
Trench volume is length x width x depth. The pipe displaces its own cylinder volume (a 4 inch pipe removes 0.087 cubic feet per foot of run), so stone needed is trench minus pipe, plus 10 percent waste. A standard 40 ft drain at 12 x 18 inches with 4 inch pipe needs about 3.1 tons of clean stone, 220 sq ft of fabric and 42 ft of pipe. Deeper curtain drains for basement water get 24 inches or more, which nearly doubles the stone.
The fabric burrito, and slope
Line the trench with non woven geotextile fabric before any stone goes in: bottom, both walls, with enough spare to fold over the top like a burrito. The fabric keeps soil fines out of the stone; without it, the drain silts solid in 5 to 10 years. Non woven is the drainage type; the woven fabric used under river rock beds passes water too slowly. Slope the pipe at a minimum 1 percent grade, which is 1 foot of fall per 100 feet of run, checked with a string line and level, and always daylight the outlet somewhere water is welcome.
Common sizing mistakes
Too narrow: a 6 inch trench barely fits the pipe plus the required 2 to 3 inches of stone around it; 12 inches is the working minimum. Too shallow: surface water needs only 12 inches, but footing drains protecting a basement belong at footing depth. Holes down: perforated pipe is laid with holes at 4 and 8 o clock positions, not straight up, so water enters as the level rises from the bottom of the trench.