How Do I Calculate Materials for a French Drain?
A french drain needs three things: drainage rock, perforated pipe, and filter fabric. Calculate the trench volume (length × width × depth), subtract the pipe volume, then divide by 27 to get cubic yards of rock. A 50-ft trench, 12 inches wide and 24 inches deep, around a 4-inch pipe needs about 4.8 tons of #57 stone, 50 ft of perforated pipe, and 275 sq ft of filter fabric. Add 10% for waste and cutting losses.
A french drain is a trench filled with drainage rock wrapped in filter fabric, with a perforated pipe along the bottom that carries water to an outlet. Building one correctly means sizing all three materials — rock, pipe, fabric — to your trench dimensions. The calculator above handles the arithmetic; the sections below explain the reasoning behind the numbers.
Formula:Rock volume (cu ft) = (Length × Width × Depth) − (π × pipe radius² × Length). Convert to cubic yards, multiply by density (1.35 tons/yd³ for #57 stone) to get tons. Pipe = trench length. Fabric = trench length × (width + 2 × depth + 0.5 ft overlap), all in feet.
How Deep Should a French Drain Be?
18–24 inches for surface drainage (yards, patio edges); 36–48 inches for foundation drains (below the footing). The pipe sits on a 2–3 inch compacted bed below the trench bottom and slopes at least 1 inch per 10 feet (1% grade) toward the outlet. Without slope, water sits in the pipe and the drain stops working.
Surface french drains for yard drainage typically run 18–24 inches deep — enough to intercept groundwater flowing through the topsoil layer without requiring heavy excavation equipment. Foundation drains must run below the footing, which means 36–48 inches on most residential slabs and deeper for basements. The single most important detail at any depth is slope: every french drain needs a minimum 1% grade (1 inch per 10 feet) toward the outlet.
How Wide Should a French Drain Trench Be?
12 inches wide is standard for a 4-inch pipe; 14–16 inches for a 6-inch pipe. You want roughly 4 inches of drainage rock on each side of the pipe — wider trenches collect more water but cost more in excavation and materials.
The goal is to surround the pipe with a generous envelope of clean rock so water can flow freely into the perforations from all sides. A 12-inch trench around a 4-inch pipe leaves 4 inches of rock on each side — the minimum for good hydraulic performance. Going narrower makes the drain easier to clog and harder to install; going wider is fine but every extra inch of width adds linear material cost across the full run.
What Size Perforated Pipe Should I Use?
4-inch corrugated perforated pipe (ADS N-12 or equivalent) handles most residential applications. Go to 6 inches for runs over 150 feet, high-flow situations, or when the drain collects concentrated roof runoff from large downspouts.
The most common perforated pipe for residential french drains is 4-inch corrugated polyethylene with a row of small slots or holes along the bottom quarter. It comes in 10-foot straight sections or 100-foot rolls, with fittings to match. Rigid PVC with drilled perforations is stiffer and easier to keep at a precise slope — worth the extra cost on foundation drains where long-term performance matters.
Orient the perforations down. This is counterintuitive but correct: water flowing into the rock envelope enters the pipe from below, and perforations-down ensures the pipe starts draining as soon as water reaches it, not only after it rises to the top half.
What Rock Should I Use for a French Drain?
Washed #57 stone (¾-inch clean) is the industry standard. It's angular, voids-rich, and free-draining. #67 stone is a fine substitute. Avoid pea gravel (too small — pipe perforations clog) and crushed concrete (fines wash into the pipe).
French drains rely on the rock's void space to move water laterally to the pipe. Washed #57 stone — nominally ¾-inch clean, angular aggregate — has about 30–40% void space by volume, which is ideal. Rounded pea gravel packs too tightly and the rounded grains pass through pipe perforations. Crushed concrete looks similar to clean stone but almost always contains fines that clog the pipe within a year or two. Pay the small premium for properly washed drainage aggregate.
Do I Need Filter Fabric?
Yes. Every french drain needs non-woven filter fabric (geotextile) wrapping the rock envelope. Without it, surrounding soil migrates into the rock voids and clogs the pipe within 5–10 years. With proper fabric, drains routinely last 30–40 years.
Filter fabric (4–8 oz/yd² non-woven geotextile) separates the drainage rock from the surrounding soil. Water passes through freely, but soil particles are held back. The fabric wraps the bottom and sides of the rock envelope, then closes at the top with a 6-inch overlap. "Landscape fabric" from a big-box store is usually woven and too coarse — specify non-woven drainage geotextile at your supplier.
Cost varies by region. The Estimated Material Cost card pulls from our indicative national-average pricing dataset(refreshed quarterly). Northeast and California metros run 15–40% above the national midpoint while Midwest and Southeast metros run 5–15% below — verify locally for binding quotes.
How Much Does French Drain Rock Cost?
DIY material cost runs $300–500 for a typical 50-ft residential french drain. Contractor-installed drains run $25–50 per linear foot. The big cost drivers are depth, access (machine vs. hand-dig), and whether the outlet requires a dry well or sump pump.
Material costs are roughly: rock $30–40/ton delivered, perforated pipe $1–2/ft, non-woven filter fabric $0.80–1.50/sq ft, plus fittings and an outlet. A 50-ft residential french drain with #57 stone, 4-inch pipe, and 4-oz fabric comes in around $300–500 in materials. Professional installation adds labor and equipment — expect $25–50 per linear foot on straightforward jobs, more if the trench runs under a driveway, crosses utilities, or requires a dry well outlet.
How Long Does a French Drain Last?
30–40 years with washed rock and non-woven fabric; 5–10 years without fabric or with the wrong kind. The failure mode is almost always soil migration into the rock envelope, not pipe degradation. Get the fabric right and the drain outlasts everything else in the yard.
Related Guides
- How to Install a French Drain— Seven-step install guide covering planning, excavation, slope, pipe, rock, fabric, and outlet
How Much Does French drain rock Cost?
French drain rock averages about $45.00 per ton nationally as of our April 2026 research. #57 (0. Regional variation is significant — Northeast and California metros run 15–40% above the national midpoint while Midwest and Southeast metros run 5–15% below. See our pricing methodology for sources and confidence tiers.
Related Calculators
- Drainage Rock Calculator— Size rock for footing drains, dry wells, and under-slab drainage
- Gravel Calculator— Estimate gravel for sub-bases and driveways
- Fill Dirt Calculator— Calculate backfill over completed drains
Written by Daniel McCarney — AceCalc