What Goes Into a French Drain
A French drain is four materials: clean drainage stone, perforated pipe, non-woven filter fabric, and an outlet — usually a dry well. The standalone French Drain Calculator sizes the trench. What it does not size is the part people forget until the trench is already dug: where the water actually goes. This list adds the outlet, because a drain that carries water 60 feet and then stops is just a very expensive hole.
Every line traces back to its standalone calculator. The trench rock, pipe, and fabric come from the French Drain Calculator; the dry-well stone comes from the Drainage Rock Calculator. Same math, not a lookalike.
A Worked Example: 60 ft Yard Drain
A 60-foot run in a 12-inch-wide, 18-inch-deep trench with 4-inch perforated pipe, discharging into a 4 × 4 foot dry well 48 inches deep:
- 4.45 tons of #57 clean stone in the trench
- 63 linear feet of perforated pipe
- 284 square feet of non-woven filter fabric
- 3.36 tons of stone for the dry well
- 7.81 tons of rock total, and 3.33 cubic yards of excavation for the trench alone
Notice the dry well is nearly as much stone as 60 feet of trench. That is the line item that blows up budgets, and it is exactly the one left out of every “how much gravel for a French drain” answer online. The pipe figure includes a 5% overage; the rock has the pipe's own displaced volume already subtracted.
The Rules That Decide Whether It Works
Slope beats depth. A French drain moves water by gravity and nothing else. Aim for about 1% fall — roughly an inch of drop every 8 to 10 feet — continuously from the high end to the outlet. A perfectly built trench with a flat spot in it holds water at the flat spot.
Use clean stone, not crusher run.#57 or #67 washed angular stone, about 3/4 inch, is the standard. Clean means no fines. The void space between the stones is the drain's actual capacity, so dense-grade aggregate, crusher run, or anything with dust in it defeats the purpose the moment it compacts.
Wrap it in non-woven fabric. Fine soil migrating into the voids is how French drains die, usually within a few years of installation. Non-woven drainage fabric passes water and blocks silt. Woven weed barrier does the opposite of what you need here.
Holes face down. Water enters the pipe from the bottom of the stone bed and is carried off immediately. Holes up means the trench has to fill before the pipe does any work at all.
Budgeting the Real Cost
Materials for a run this size are a few hundred dollars. Excavation is the expense. Digging 3.3 cubic yards of trench with a shovel is a long, miserable weekend, and that figure does not include the dry well, which is another 2.4 cubic yards of hole on its own. On any run past about 40 feet, a one-day mini-excavator rental usually costs less than the weekend it saves — and gives you a straighter, more consistent trench bottom, which is what the slope rule depends on.
Before you dig anything, call 811 (or your local utility locate service) and have the yard marked. A drain trench at 18 inches is exactly the depth where irrigation lines, low-voltage lighting, and shallow utility runs live.
Export Your List
Use the buttons above the article to copy the list, download a PDF, or email it to yourself. When supplier links are configured, the “Send this list to Home Depot” section opens a live, in-stock search for every line item — one click per material.
Related
- French Drain Installation Guide — step-by-step build
- Dry Well Installation Guide — sizing and building the outlet
- French Drain Calculator — just the trench
- Drainage Rock Calculator — stone tonnage on its own
- All project materials lists
Written by Daniel McCarney — AceCalc