French Drain: Materials, Cost & Calculator (2026 Guide)

One common way to manage subsurface yard water is to dig a trench, fill it with rock and perforated pipe, and give the water somewhere to go. That's a French drain. The concept is simple. The math is not complicated. But most homeowners either buy too little gravel (and have to make a second trip) or buy way too much (and are left with a pile they don't need).

This guide covers what a French drain is, how to size the trench, how much gravel and pipe you need, what it costs, and what mistakes to avoid — with a free calculator to do the math for you.

What Is a French Drain?

A French drain is a trench with perforated pipe surrounded by rock and filter fabric — as PNNL / DOE Building America describes it — that redirects subsurface water away from a problem area. Water flows into the rock, down to the pipe, through the pipe, and out to a suitable discharge point.

Identify the outlet before digging.The pipe needs positive fall to a suitable discharge point: daylight at a lower part of the property, a properly designed drywell or infiltration system where site soils, groundwater conditions and local rules make infiltration appropriate, an approved stormwater system, or another locally permitted point. If discharging to daylight, direct the outlet away from buildings and protect it from erosion. Stormwater discharge rules vary — check local requirements before connecting to a storm sewer or directing runoff off-site, and do not discharge onto a neighbor's property, a sidewalk or a street without local approval.

Despite the name, it has nothing to do with France. It's named after Henry French, a Massachusetts farmer who popularized the technique in the 1800s.

French drains are used for:

• Soggy or chronically wet yards
• Intercepting subsurface water uphill of a problem area (not a substitute for foundation drainage — see below)
• Driveway edge runoff
• Drainage behind retaining walls
• Redirecting downspout water

A French drain is best at intercepting subsurface water and some localized surface drainage; concentrated surface runoff may be better handled with grading, swales or surface inlets. If you're weighing it against a channel drain (which intercepts surface runoff at grade), Basement Repair Guide's French drain vs. channel drain comparison covers the key differences for homeowners deciding which type fits their situation.

French Drain Calculator

Enter your trench length, width, and depth below. The calculator gives you cubic yards of drainage gravel, tons, landscape fabric requirements, pipe length, and a material cost estimate.

French Drains Are Not Foundation Drainage

Foundation drainage is a separate, code-sensitive application. A footing/perimeter drain is coordinated with footing elevation, waterproofing and a suitable outlet or sump. A generic shallow yard French drain should not be treated as a substitute for foundation drainage or waterproofing — see PNNL's footing-drain guide. Don't excavate next to an existing foundation without qualified guidance. Have underground utilities located before digging (811 in the US) and check local excavation requirements.

The Formula

French drain gravel is calculated like any fill-in-a-trench project:

Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards

Convert width and depth from inches to feet first (divide by 12). Then multiply all three dimensions to get cubic feet, and divide by 27 to get cubic yards. Multiply cubic yards by 1.4 to get approximate tons for crushed stone drainage gravel.

For a 50 ft trench at 12" wide × 18" deep:

50 × (12÷12) × (18÷12) ÷ 27 = 50 × 1 × 1.5 ÷ 27 = 2.78 yd³ of trench volume

The calculator subtracts the small volume the pipe occupies (about 0.16 yd³ for 4" pipe), so its default result for this trench is about 2.62 yd³. At 1.4 tons per cubic yard, that's about 3.7 tons of rock.

French Drain Trench Sizes: Planning Examples

There is no single standard trench. Trench width depends on pipe size, aggregate volume and the drainage problem; depth depends on what water you are trying to intercept, site grade and the outlet elevation. NDS notes many water-diverting French drains fall roughly in the 8-inch to 2-foot range, while foundation and retaining-wall systems can be deeper. PNNL's detailed residential example uses a trench at least about 22 inches deep and 14 inches wide for a 4-inch pipe, placing the drain about 18 inches or more below grade — one installation detail, not a universal standard. The table below shows common planning examples, not requirements:

Gravel: What to Use and How Much

Not all gravel drains well.

Use clean, free-draining rock with minimal fines.Clean stone with no dust or crushed fines leaves air gaps between pieces so water flows through easily. Avoid soil, crushed fines or dense-graded road base immediately around the perforated pipe — they clog. Suppliers sell this under names such as "clean crushed stone" or "drainage gravel," and the size designation varies by supplier.

In PNNL's residential installation detail, the rock is clean and roughly ¾–2 inches in size, with about 4 inches of rock below the pipe and about 4–6 inches over it. That is one detail, not a requirement for every drainage product — follow the selected drainage system's instructions.

Crushed stone for drainage weighs approximately 1.35–1.45 tons per cubic yard (the calculator uses 1.4 tons/yd³). Use our gravel calculator if you just need a quick cubic yards or tons estimate without the fabric and pipe calculations.

Consider ordering about 10% more gravelthan the calculator shows as a planning allowance for irregular trench walls, minor spillage and estimating uncertainty. It is not an engineering standard, and the calculator's result does not include it.

Gravel Needed by Trench Size (Trench Volume)

Landscape Fabric: How Much Do You Need?

Use permeable filter fabric/geotextile intended for drainage to separate surrounding soil from the clean aggregate and reduce migration of fines into the drain. It lines the inside of the trench before the rock goes in; not every retail "landscape fabric" or weed barrier is appropriate, so choose a product rated as a drainage filter fabric. A prewrapped/filter-sock pipe can help keep sediment from the pipe openings, while wrapping the aggregate zone with suitable filter fabric also separates surrounding soil from the drainage stone — follow the selected drainage-system detail.

The fabric drapes down all four sides of the trench in a U-shape, wraps over the pipe and gravel, and overlaps at the top. To calculate the fabric width you need:

Fabric width = trench width + (2 × trench depth) + 2 ft overlap

For a 12" wide × 18" deep trench: 1 + (2 × 1.5) + 2 = 6 ft wide, so a 6-ft wide roll. For length, add 2 ft to your trench length for end overlaps.

Landscape fabric is inexpensive (typically $0.20–$0.55 per linear foot for standard 4-ft wide rolls; wider rolls cost slightly more per foot but save cutting time).

Perforated Pipe: 4 Inch vs 6 Inch

A 4-inch perforated pipe is a common residential starting point. NDS says it is likely adequate for many residential applications, but heavy saturation or larger collection areas can require larger pipe.

Consider larger pipe (the calculator offers 6-inch) when, for example:

• You have a high water table or heavy saturation
• You're collecting water from a large roof or impervious area
• Multiple downspouts tie into the same trench

Drain capacity depends on pipe diameter, slope, inlet conditions, soil, aggregate and the amount of water reaching the system. YardCalc estimates materials and geometry; it does not perform stormwater hydraulic design.

Buy pipe length equal to your trench length, plus 5% for connectors and end caps. 4" perforated pipe costs $1.50–$3.50 per foot; 6" pipe runs $2.50–$5.00 per foot.

For conventional perforated PVC like the PNNL detail, install the perforations down. Other slotted or proprietary drainage products can use different opening patterns, so follow the pipe manufacturer's installation instructions. Don't drill or modify pipe.

Both perforated PVC and corrugated drainage products are used. PNNL prefers smooth PVC in its residential detail because corrugations are more likely to retain sediment; follow the selected product's installation requirements.

Cost Breakdown

Here's what the calculator shows for a 50 ft French drain (12" wide × 18" deep with 4" pipe) at its current unit-price assumptions; local prices will vary:

Professional installation adds significant labor cost. Expect $20–$50 per linear foot installed — roughly $1,000–$2,500 for a 50 ft drain. That price covers digging, materials, installation, and disposal of excavated soil.

DIY is very achievable for most homeowners with a standard yard. The main equipment you need: a trench shovel or rented trencher ($150–$350/day), a level or laser level, and basic hand tools.

French Drain Cost Per Linear Foot

Here's a quick reference for what a French drain costs per linear foot, depending on whether you DIY or hire a contractor:

Material cost is largely driven by gravel price, which varies by region. Check with local landscape supply yards for current crushed stone pricing. Use our gravel calculator for a standalone gravel estimate if you're sourcing materials separately.

Common Mistakes to Avoid

• Skipping utility locates. Have underground utilities located before digging (811 in the US) and check local excavation requirements.
• No positive fall. No fall means no flow. Measure your grade and outlet elevation before digging; about 1% (roughly 1 foot of drop per 100 feet) is a useful planning target when the site allows, but not a universal minimum.
• No suitable outlet. The water has to go somewhere. A French drain with no outlet just moves water before it pools again. Outlets can include daylight at a lower part of the property, a drywell where soils and local rules make infiltration appropriate, or an approved stormwater system — stormwater discharge rules vary, so check locally.
• Rock with fines.Soil, crushed fines and dense-graded road base around the pipe clog the drain. Use clean, free-draining rock with minimal fines that meets your drainage system's requirements.
• Skipping or misusing fabric. Without suitable filter fabric, fine soil particles can migrate into the rock and clog the drain. Use permeable geotextile intended for drainage, not an impermeable weed barrier.
• Undersizing the pipe.A 4" pipe is a common residential starting point, but heavy saturation or larger collection areas can require larger pipe.

Frequently Asked Questions

How much gravel do I need for a French drain?

Length (ft) × width (ft) × depth (ft) ÷ 27 = cubic yards. A 50 ft trench at 1 ft × 2 ft needs about 3.7 cubic yards (~5 tons).

How deep should a French drain be?

It depends on the water source, site grade and outlet elevation. NDS notes many water-diverting French drains fall roughly in the 8-inch to 2-foot range; PNNL's residential detail uses a trench at least about 22 inches deep. The pipe needs positive fall toward a suitable outlet.

What gravel is best for a French drain?

Clean, free-draining rock with minimal fines, with the aggregate zone wrapped in permeable filter fabric/geotextile intended for drainage. PNNL's residential detail uses clean rock roughly ¾–2 inches in size.

How long does a French drain last?

30+ years is possible with proper installation and maintenance; actual service life varies with soil conditions, sediment, roots and materials. Without suitable filtration, fine soil can migrate into the aggregate and pipe openings, increasing the risk of clogging. Keep the outlet clear and inspect the system periodically.

Related Calculators & Guides

If you're working on yard drainage or a gravel project, these calculators are useful alongside the French drain calculator:

• Gravel Calculator — for patios, driveways, and general landscaping gravel estimates
• Fill Dirt Calculator — for grading and leveling around your drain outlet, or backfilling after installation
• Retaining Wall Calculator — French drains are often essential alongside retaining walls for proper backwall drainage
• How Much Fill Dirt Do I Need? — covers grading and drainage planning for yards
• How to Build a Retaining Wall — includes drainage requirements for retaining walls

When you're ready to source materials, #67 limestone and pea gravel can both be ordered in bulk online — almost always cheaper per ton than bags for a project this size. Use code MEADOWLARK for 5% off your first bulk order.