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

Most yard drainage problems are solved the same way: dig a trench, fill it with gravel and 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 gravel-filled trench — usually containing a perforated pipe — that redirects groundwater and surface water away from a problem area. Water flows into the gravel, down to the pipe, through the pipe, and out to a discharge point (a ditch, a dry well, a low area away from your house).

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
• Water pooling against a foundation or basement wall
• Driveway edge runoff
• Drainage behind retaining walls
• Redirecting downspout water

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.

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

At 1.4 tons per cubic yard, that's about 3.9 tons of crushed stone.

Standard French Drain Trench Sizes

The right trench dimensions depend on how much water you need to move and how serious the drainage problem is. Here are standard sizes:

Gravel: What to Use and How Much

The gravel choice is where most DIYers go wrong. Not all gravel drains well.

Use crushed stone #57 or washed drainage gravel.These are angular, clean stones (no fines, no dust) that stack loosely with air gaps between them. Water flows through easily. They're also called "clean crushed stone," "drainage gravel," or "#8 stone" at some suppliers.

Do not use pea gravel or round river rock.Round stones don't interlock — they shift and compact over time, reducing drainage. Some pea gravel is fine for aesthetics on top, but not as the functional fill layer in a drainage trench.

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.

Always order 10% more gravel than your calculation shows. Trench walls are never perfectly smooth, and gravel settles over time.

Gravel Needed by Trench Size

Landscape Fabric: How Much Do You Need?

Landscape fabric (filter fabric or geotextile) lines the inside of the trench before the gravel goes in. It lets water pass through but keeps soil particles out — without it, fine soil slowly migrates into the gravel and clogs the drain over time.

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 = 5 ft wide. Buy 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

For most residential drainage problems, a 4-inch perforated pipe is sufficient. It handles the typical runoff from a wet yard, a downspout, or a driveway edge.

Use a 6-inch pipe when:

• You have a high water table
• You're managing runoff from a large roof or impervious area
• The drain is near a foundation and failure would be costly
• Multiple downspouts tie into the same trench

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.

Lay the pipe holes-down (perforation side facing down). This is counterintuitive — you might think you want holes on top to catch water — but holes-down means water that saturates the gravel can flow up through the holes into the pipe and away, rather than letting the pipe fill with sediment from above.

Cost Breakdown

Here's what to expect for a standard 50 ft French drain (12" wide × 18" deep with 4" pipe):

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

• Not calling 811. Always call 811 (the national dig safe hotline) before digging. Underground utilities are the most common serious French drain mistake.
• Insufficient slope. No slope = no flow. Measure your grade before digging. You need at least 1 inch of drop for every 8 feet of run.
• No discharge point. The water has to go somewhere. A French drain with no outlet just moves water 20 feet before it pools again. Outlets: daylight on a slope, a dry well, a drainage swale, or storm drain (check local codes).
• Wrong gravel. Pea gravel, landscape rock, and crushed concrete are not substitutes for proper drainage gravel. Use clean crushed stone (#57 or #8) only.
• Skipping the fabric. Without filter fabric, fine soil particles will gradually migrate into the gravel and clog the drain within a few years. Fabric is cheap insurance.
• Undersizing the pipe.A 4" pipe works for most applications, but in high-volume situations, it will back up. When in doubt, go 6".

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