The short version
For a standard 18" deep × 12" wide French drain: 1 ton of 3/4" washed rock per 10 linear feet, plus 4" perforated corrugated pipe (sock optional), and non-woven landscape fabric wide enough to wrap the trench like a burrito. That's the working recipe.
The math
A French drain is a gravel-filled trench with a perforated pipe at the bottom that catches groundwater and moves it somewhere useful (the curb, a dry well, the back of the property). On DFW clay soils where water sits on top of the ground for two days after a storm, it's the most useful drainage tool you have.
Trench dimensions — pick one:
| Trench | Depth × Width | Rock per linear foot |
|---|---|---|
| Light yard drain (kids' swing set drains) | 12" × 8" | 0.05 cu yd / 0.07 ton |
| Standard yard drain (most uses) | 18" × 12" | 0.10 cu yd / 0.14 ton |
| Foundation perimeter drain | 24" × 12" | 0.13 cu yd / 0.18 ton |
| Heavy commercial / driveway crossing | 24" × 18" | 0.20 cu yd / 0.28 ton |
For 50 linear feet of standard 18" × 12" drain:
50 ft × 0.14 ton/ft = 7 tons of 3/4" washed rock
Bump that to 8 tons to allow for the bowl where the rock ends, settling after the first rain, and the cap layer over the pipe. Better to have leftover gravel for the next project than to be 1/4 ton short with a half-buried trench.
Pipe — 4" perforated corrugated pipe is the workhorse. Sold in 10-foot sticks (rigid) or 100-foot rolls (flexible). For straight runs over 30 feet, use rigid; it doesn't sag and it's easier to snake a camera through later. For runs that turn around corners, use flexible. Sock or no sock: if your soil has any silt content (most of DFW does), pipe with a fabric sock filters fines before they clog the perforations. Skip the sock only if you're also wrapping the entire trench in fabric.
Fabric — non-woven landscape fabric, 4-6 oz weight. For an 18×12 trench, you want fabric at least 5 feet wide so you can line all four sides and overlap the top. Calculate: trench length × 5 ft.
50 ft × 5 ft = 250 sq ft of fabric → one 4 ft × 100 ft roll plus overlap, or a 6 ft × 100 ft roll.
Slope — at least 1% (1" drop per 100"). Anywhere from 1-2% works on a flat lot; 2-3% if you have the elevation. Without slope, the drain holds water like a bathtub.
Common mistakes
- Using crushed limestone instead of washed rock. Limestone has fines that pack and seal off the pipe's perforations. Always use clean, washed 3/4" rock (sometimes called "drainage rock" or "57 rock"). The fines are washed off at the quarry.
- Skipping the fabric. A French drain without fabric is a 1-season project. Silt and clay migrate in, fill the air gaps in the rock, and the drain stops working. Wrap it.
- Running uphill. Sounds obvious. People do it because they put the outlet where it looks nicest instead of where gravity actually points. Use a string line and a level before you order rock.
- Outletting onto your neighbor's yard. That's a phone call you don't want. Outlet to the curb, the street drain, or a dry well on your own property.
What to do next
For a step-by-step that sizes rock, pipe, fabric, and outlet drain for your specific run, the French-drain wizard on the rock & gravel page walks you through it in five questions and builds the order. To shop the materials directly, see rock & gravel for the 3/4" washed and accessories for perforated pipe, sock pipe, and non-woven fabric by the roll.
Also useful
- How much rock or gravel do I need — the tonnage math, same units
- How to measure a flagstone or paver patio — same trenching mindset for hardscape base