French drain calculator
A 50 ft French drain in a trench 12 in wide and 18 in deep, with 4 in of soil on top, takes about 2.97 tons of drain stone (2.20 cubic yards), 50 ft of 4 in perforated pipe and 272 sq ft of filter fabric.
How big to make the trench
A French drain is a trench lined with filter fabric, holding a perforated pipe in washed stone, that carries water away from a wet spot or a foundation. Louisiana State University's AgCenter describes a trench 12 to 18 in wide and 18 to 24 in deep with a 4 in pipe; the City of Norfolk's homeowner guide calls for at least 12 in wide and 18 to 24 in deep. The University of Georgia's demonstration drain is 18 in by 18 in. A geotextile maker's guide puts the narrowest practical trench at about 10 in, a little wider than a shovel, and notes that a 12 in backhoe bucket digs 13 to 15 in.
| Trench, wide × deep | 10 ft | 50 ft | 100 ft |
|---|---|---|---|
| 12 × 18 in | 0.54 | 2.70 | 5.40 |
| 12 × 24 in | 0.79 | 3.95 | 7.90 |
| 18 × 18 in | 0.83 | 4.16 | 8.31 |
| 18 × 24 in | 1.21 | 6.03 | 12.06 |
50 ft × (1 ft × 1.167 ft − 0.087 sq ft of pipe) × 1.1 = 59.37 cu ft ÷ 27 = 2.20 cu yd
2.20 cu yd × 2,700 lb ÷ 2,000 = 2.97 tons
Stone, pipe and fabric
Line the bottom and sides with nonwoven filter fabric, leaving enough at the top to fold over the stone. Lay 2 to 4 in of stone, set the pipe on it with the holes facing down, and fill with stone to within a few inches of the surface: Norfolk's guide stops 3 to 5 in short, a geotextile maker's guide 6 in. Fold the fabric over the stone, overlapping it, and finish with soil and sod or with decorative stone. Where two pieces of fabric meet along the trench, overlap them by at least 12 in, with the upstream piece on top.
The stone is washed, so water moves through it freely: ½ to ¾ in in Norfolk's guide, about 1 in in LSU's. The calculator weighs it as loose crushed stone at 2,700 lb a cubic yard and takes out the space the pipe fills. The fabric strip it gives is wide enough to cover the bottom, both sides and the top of the stone with 12 in to spare.
| Item | Typical | Range | Stores |
|---|---|---|---|
| 4 in × 10 ft perforated PVC drain pipe | $20.60 | $16.49 to $23.99 | 8 |
| 4 in × 100 ft perforated corrugated pipe with filter sock | $132.18 | $77.99 to $159.99 | 8 |
Slope and outlet
The trench has to fall steadily toward the place the water will go. The University of Georgia recommends at least 1%, about ⅛ in a foot; Norfolk's guide asks for 1 in every 8 ft, and Washington State University Extension 0.5 to 1%. A 50 ft drain drops at least 6 in. Run the pipe out to daylight on lower ground, a dry well or a storm drain where that's allowed, and not onto a neighbor's lot. Mark the route and call 811 before you dig.
For a drain dug by a contractor, French drain cost gives the price per foot, and gravel prices what stone costs by the ton in 2025 price lists.
What you'll need
- A trenching shovel has a long, narrow blade that cuts a clean trench without widening it.
- A string line and line level set the fall: stretch the line along the route and measure down to the trench floor every few feet.
- Nonwoven filter fabric is what the guides call for: it lets water through and keeps soil out of the stone.
- Landscape fabric staples hold the fabric against the trench walls while you fill.
- A contractor wheelbarrow moves the stone in and the spoil out.
Sources
- Installing and maintaining a French drain, LSU AgCenter, 12 November 2025. A trench 12 to 18 in wide and 18 to 24 in deep, a 4 in perforated pipe and about 1 in washed gravel, with geotextile lining the bottom and sides.
- Infiltration: French drains, trenches and dry wells, City of Norfolk, Virginia, read November 2026. A trench at least 12 in wide and 18 to 24 in deep; slope 1 in every 8 ft; 2 to 3 in of gravel under the pipe, gravel to 3 to 5 in below the top; fabric overlapped at least 12 in.
- Stop 2: French drain, University of Georgia, Green Stormwater Infrastructure demonstration site, read November 2026. An 18 × 18 in trench, a 4 in pipe 3 in above the bottom, nonwoven geotextile, at least 1% slope; $20 to $50 or more a linear foot with materials and labor.
- Improving drainage, Washington State University Extension, Clark County, read November 2026. French drain trench sloped 0.5 to 1%.
- Installation guide for a backyard geotextile French drain, US Fabrics, read November 2026. A trench at least about 10 in wide; 4 in of stone under the pipe; stone to 6 in below grade; at least ½% slope.
- Maintenance Manual Volume 1, Appendix: Useful Information, California Department of Transportation, July 2014. Loose weights of crushed gravel, limestone, granite, screenings, sand and soil; 111 lb of asphalt mix per square yard per inch; 2.0 tons per compacted cubic yard of base.
- Highway Maintenance Tables and Conversion Factors (Engineering Policy Guide appendix), Missouri Department of Transportation, current guide. Loose and compacted weights of crushed stone, sand, asphalt and concrete in pounds per cubic foot and per cubic yard.
- Online store listings for bagged concrete, mortar, grout, block, rebar, mesh, gravel, sand, soil, mulch and pavers, Google Shopping results (Home Depot, Lowe's, Menards, Ace Hardware, Walmart and others), November 2026. Single-item prices by store; the calculators use the median as the default and show the range.
- 2025 price list, Rock Hard Landscape Supply (Minnesota), 2025. Class 5 $34 a ton; delivery $85 to $105 for a single axle (5 tons) and $125 to $140 for a quad or quint axle (17 to 20 tons); fabric 4 × 100 ft $48.