French Drain Calculator
A French drain is a gravel order pretending to be a plumbing job. The trench is easy to size, but the stone that fills it has to account for the pipe running through it, and almost nobody subtracts that volume, so they over-buy rock and under-buy fabric. This tool does the geometry properly. Enter the trench length, width, and depth, pick your pipe size, and you'll get washed drain rock in tons and yards with the pipe deducted, the 50-lb bag count, and the filter fabric to line the trench.
A trench, a pipe, and honest geometry
French drain guides argue about a lot, but the material math is fixed: trench volume, minus pipe volume, equals rock. Skipping the subtraction over-orders stone by 5 to 10%, and using the wrong stone kills the drain entirely. This tool handles the first problem automatically and nags you about the second, because washed rock versus crusher run is the difference between a drain and a buried wet spot.
Rock per foot of trench
With 4-inch pipe deducted, in washed #57 stone at 1.4 tons per yard.
| Trench (W x D) | Rock per foot | Tons per 50 ft | Use |
|---|---|---|---|
| 6 in x 12 in | 0.41 cu ft | 1.1 | Downspout runs, small surface drains |
| 12 in x 18 in | 1.41 cu ft | 3.7 | Standard yard drain |
| 12 in x 24 in | 1.91 cu ft | 5.0 | Wet lawns, heavier flows |
| 18 in x 24 in | 2.91 cu ft | 7.6 | Foundation perimeter, curtain drains |
The build that keeps draining
Working order: dig with consistent fall (1% minimum, an inch every 8 feet, checked with a string line, not an eyeball). Line the trench with non-woven filter fabric, leaving enough to lap over the top. A few inches of rock, then the perforated pipe, holes facing down, because water enters from below as the trench fills. Rock to within a few inches of grade, fold the fabric over like a burrito, then topsoil and grass, or more decorative stone if you want the drain visible. The fabric burrito is the whole trick: it keeps soil from silting the voids, which is how French drains die young.
A soggy-lawn run, worked
Fifty feet from the low spot to a daylight outlet at the curb, trenched 12 by 18 with 4-inch pipe: 75 cubic feet of trench, minus 4.4 for the pipe, plus 10% waste is about 2.9 yards, 4 tons of washed stone, plus 250 square feet of fabric, one 4-inch perforated pipe run, and a pop-up emitter at the end. At typical prices that's $180 to $300 in rock, $60 in fabric, and $80 in pipe parts: a $3,000 contractor problem for under $500 in materials and a hard weekend.
Mistakes that flood basements anyway
- Crusher run in the trench. Fines pack solid and water rolls over the drain. Washed round stone or clean #57 only, the same rule the stone calculator flags.
- No outlet. A French drain isn't storage; it needs daylight, a dry well, or a sump. Water with nowhere to go finds your foundation.
- Flat spots in the run. One sag collects silt and becomes a dam. String line, stakes, and patience.
- Holes up. Perforations face down. It reads backwards; it isn't.
Common questions
A 12 by 18 inch trench with 4-inch pipe takes about 1.4 cubic feet, roughly 80 pounds, of rock per foot. So a 25-foot run is about a ton. Wider or deeper trenches scale up fast; enter yours above.
Washed, rounded stone or clean #57, 3/4 to 1.5 inches. Never crusher run or anything with fines: the dust packs the voids and the drain stops draining. Washed matters as much as size.
18 to 24 inches for surface water around lawns, deeper if you're protecting a foundation footing. Keep a consistent 1% fall, about an inch every 8 feet, toward the outlet.
Yes, line the trench before the rock goes in and lap it over the top. Fabric keeps soil from silting into the stone, which is the main way these drains die. Skip it and plan on redigging in five years.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.