French Drain Materials Calculator
Enter the trench run, size, and pipe diameter.
Landscaping
French Drain Calculator
Quick answer: drain gravel (yd³) = (trench length × width × depth − pipe volume) ÷ 27, with pipe volume = length × π × radius². A 50 ft trench 12 in wide and 18 in deep around a 4 in pipe holds 2.62 yd³ of stone — about 4.0 tons of #57 with 10% extra.
Plan a French drain from one set of trench dimensions: washed drain rock in cubic yards and tons, filter fabric to wrap the trench, 10 ft perforated pipe sections, the total fall needed from start to outlet, and the soil you will dig out and haul away.
French drain cross-section
How to calculate French drain materials
A French drain is a gravel-filled trench with a perforated pipe near the bottom, usually wrapped in non-woven filter fabric so silt cannot clog the stone. The stone fills whatever space the pipe does not, so the gravel order is the trench volume minus the pipe volume. Everything else — fabric, pipe, fall, spoil — comes from the same three trench dimensions.
(L × W × D − L × π × r²) ÷ 27
= cubic yards of gravel (all dimensions in feet, r = pipe radius)
- Measure the run from the high end of the drain to the outlet — a daylight point, dry well, or approved storm connection.
- Find the trench volume: length × width × depth in feet. 12 in = 1 ft, 18 in = 1.5 ft, 24 in = 2 ft.
- Subtract the pipe: length × 3.1416 × radius². A 4 in pipe has a 2 in (0.167 ft) radius, so it displaces about 0.087 ft³ per foot.
- Convert and weigh: ÷ 27 for cubic yards, add 10% for settling and uneven trench walls, then × 1.40 tons/yd³ for washed #57.
- Size the fabric: strip width = 2 × depth + 2 × width + overlap; area = strip width × length.
- Set the fall: length × slope. At 1% that is 0.12 in per foot, close to the 1/8 in per foot rule of thumb.
Worked example: 80 ft drain along a soggy side yard
The trench is 80 ft long, 12 in wide, and 24 in deep, with a 4 in perforated pipe, 1% fall, and a 12 in fabric overlap.
- Trench: 80 × 1 × 2 = 160.0 ft³; pipe: 80 × 3.1416 × 0.1667² = 7.0 ft³
- Gravel: 153.0 ft³ ÷ 27 = 5.67 yd³; +10% = 6.23 yd³ × 1.40 = 8.7 tons
- Fabric: 2 × 24 + 2 × 12 + 12 = 84 in (7 ft) strip × 80 ft = 560 ft² — a roll at least 7 ft wide does it in one piece; a 6 ft roll needs two overlapping strips.
- Pipe: 80 ÷ 10 = 8 sections; fall: 80 × 1% = 9.6 in lower at the outlet than at the start.
- Spoil: 160 ft³ = 5.93 yd³ in the ground, about 7.4 yd³ loose at a 1.25 swell factor.
Drain gravel and fabric per 10 feet of trench
Net stone after subtracting the pipe, before any extra %, at 1.40 tons/yd³. Fabric strip includes a 12 in top overlap.
| Trench (W × D) and pipe | Trench ft³ | Gravel ft³ | Gravel yd³ | Tons | Fabric ft² (strip width) |
|---|---|---|---|---|---|
| 8 × 18 in, 3 in pipe | 10.0 | 9.5 | 0.35 | 0.49 | 53 (5.3 ft) |
| 12 × 18 in, 4 in pipe | 15.0 | 14.1 | 0.52 | 0.73 | 60 (6 ft) |
| 12 × 24 in, 4 in pipe | 20.0 | 19.1 | 0.71 | 0.99 | 70 (7 ft) |
| 12 × 24 in, 6 in pipe | 20.0 | 18.0 | 0.67 | 0.94 | 70 (7 ft) |
| 18 × 24 in, 4 in pipe | 30.0 | 29.1 | 1.08 | 1.51 | 80 (8 ft) |
| 18 × 24 in, 6 in pipe | 30.0 | 28.0 | 1.04 | 1.45 | 80 (8 ft) |
French drain tips and common mistakes
- Call 811 before you dig. Gas, electric, cable, and irrigation lines can sit within the top 2 ft of soil — the same depth as a typical drain. The locate service is free, and most states require a request before any digging.
- Use clean stone only. Washed #57 or 3/4 in clean gravel keeps open voids. Crusher run and other dense-graded stone contain fines that compact and block water. Ask the yard for "washed" or "clean" stone by name.
- Bed the pipe, then cover it. Put 2–3 in of gravel under the pipe so it sits in the stone, not on soil, and keep several inches of stone over it. Lay the first gravel before the pipe, or the pipe ends up on the trench floor.
- Check fall with a level, not by eye. Set a string line with a line level and measure down to the trench floor every 10 ft; at 1% each 10 ft should be 1.2 in deeper. Low spots hold water and silt.
- Plan the outlet first. The drain must discharge somewhere lower — a daylight outlet with a grate, a dry well, or a storm connection where your city allows it. Do not aim it onto a neighbor's lot.
- Fabric goes around the gravel, not just over the pipe. Line the trench walls and bottom, fill, then fold the overlap across the top. A sock on the pipe alone can clog in fine silty soils.
- Budget for spoil. Dug soil fluffs up by roughly 20–30%. Reuse the topsoil to patch the lawn and haul off the rest.
French drain FAQ
How much gravel do I need for a French drain?
Multiply trench length × width × depth in feet, subtract the pipe volume (length × 3.1416 × pipe radius in feet squared), and divide by 27 for cubic yards. A 50 ft trench 12 in wide and 18 in deep with a 4 in pipe needs about 2.62 cubic yards, or roughly 4 tons of #57 stone with 10% extra.
How deep and wide should a French drain be?
Most residential French drains are 9 to 12 inches wide and 18 to 24 inches deep. Shallow interceptor drains for surface water can be about 12 inches deep, while drains meant to lower groundwater along a foundation are dug deeper. Leave 2 to 3 inches of gravel under the pipe and several inches over it.
How much slope does a French drain need?
A fall of 1 percent, about 1/8 inch per foot, is the common minimum, and 2 percent drains more reliably. At 1 percent a 50 ft drain drops 6 inches from start to outlet, and a 100 ft drain drops 12 inches.
What size gravel is best for a French drain?
Clean, washed angular stone about 3/4 inch to 1 inch, such as #57, is the usual choice because it has open voids for water to flow through. Avoid crusher run, rock dust, or any stone with fines, which pack tight and slow drainage. Washed #57 weighs about 1.35 to 1.45 tons per cubic yard.
Should French drain pipe holes face up or down?
For rigid perforated pipe with two or three rows of holes, the common recommendation is holes facing down, so water rising in the trench enters from below and the pipe carries it away without leaving a pool in its bottom. Fully slotted corrugated pipe can be laid in any orientation.
How much filter fabric do I need for a French drain?
Cut a strip as wide as two trench depths plus two trench widths plus an overlap of about 12 inches, so the fabric lines both walls and the bottom and folds over the top. A trench 12 in wide and 18 in deep needs a 6 ft wide strip, or 300 square feet for a 50 ft run.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.