French Drain Calculator
Calculate gravel, landscape fabric, and pipe for your French drain project. Get cubic yards, tons, and a material cost estimate.
Quick Answer
A 50 ft French drain trench at 12" wide × 18" deep needs about 2.62 yd³ of drainage gravel (~3.7 tons), 52 linear ft of landscape fabric, and 53 ft of 4" perforated pipe — totaling roughly $181–$382 in materials at the calculator's current unit-price assumptions.
Planning Trench Width
Trench width depends on pipe size, aggregate volume and the drainage problem.
Planning Trench Depth
The right depth depends on the water source, soil, outlet elevation and application.
A 4" perforated pipe is a common residential starting point; heavy saturation or larger collection areas can require larger pipe. YardCalc estimates materials and geometry — it does not perform stormwater hydraulic design.
Enter your trench length to calculate materials.
💡 Consider ~10% extra gravel as a planning allowance for irregular trench walls and estimating uncertainty (not included in the result). Have underground utilities located (811 in the US) and check local excavation requirements before digging.
Slope Check
Planning slope: about 1% (roughly 1" of drop per 8 ft of run) is a useful residential planning target. The drain ultimately needs positive fall to its outlet.
Drop at selected slope
Enter a run distance to see the drop at 1% and 2% slopes.
How to Calculate French Drain Materials
Formula
Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards
Convert trench width and depth from inches to feet before multiplying. Divide the total cubic feet by 27 to get cubic yards. Multiply cubic yards by 1.4 to get tons of drainage gravel (clean, free-draining rock with minimal fines). Consider ordering about 10% extra 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.
Example
A 50 ft trench that is 12" wide (1 ft) × 18" deep (1.5 ft): 50 × 1 × 1.5 ÷ 27 = 2.78 yd³of trench volume. The calculator subtracts the small volume the pipe occupies (about 0.16 yd³ for 4" pipe), so its result is about 2.62 yd³. At 1.4 tons per yd³, that's about 3.7 tons of rock. The landscape fabric wraps the full trench interior — for this trench, you'd need fabric at least 6 ft wide (1 ft bottom + 1.5 ft each side + 2 ft total overlap) and 52 linear feet.
Tips & Best Practices
- •Have underground utilities located before digging (811 in the US) and check local excavation requirements.
- •Use clean, free-draining rock with minimal fines. Avoid soil, crushed fines or dense-graded road base around the pipe.
- •A French drain needs positive fall toward a suitable outlet. About 1% (roughly 1 foot of drop per 100 feet) is a useful planning target when the site allows, but it is not a universal code minimum; pipe type, site grade and outlet elevation can justify a different slope.
- •Use permeable filter fabric/geotextile intended for drainage to separate surrounding soil from the clean aggregate and reduce migration of fines into the drain.
- •A 4" perforated pipe is a common residential starting point; heavy saturation or larger collection areas can require larger pipe.
- •Identify the outlet before digging. The pipe needs positive fall to a suitable discharge point, and local stormwater rules may control where water can go.
- •Consider ~10% extra gravel as a planning allowance for irregular trench walls, minor spillage and estimating uncertainty.
Outlets and discharge. Potential outlets include 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, or an approved stormwater system. 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 street without local approval. See the PNNL / DOE Building America guide for outlet examples.
Not a foundation fix. 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.
Right tool for the water. French drains are useful for intercepting subsurface water and some localized surface drainage, but concentrated surface runoff may be better handled with grading, swales or surface inlets. Drain capacity depends on pipe diameter, slope, inlet conditions, soil, aggregate and the amount of water reaching the system.
Pipe, fabric and rock details.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. 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. 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. In PNNL's residential detail the rock is clean, roughly ¾–2 inches, with about 4 inches below the pipe and 4–6 inches over it; that is one installation detail, not a universal requirement.
Depth, width and sizing context. French-drain 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 drainage systems can be deeper. PNNL's residential detail places the drain about 18 inches or more below grade in a trench at least about 22 inches deep and 14 inches wide for a 4-inch pipe — one installation detail, not a universal standard.
Gravel Needed by Trench Size
| Trench Size | Per 10 ft | Per 50 ft | Per 100 ft |
|---|---|---|---|
| 6" wide × 12" deep | 0.19 yd³ | 0.93 yd³ | 1.85 yd³ |
| 12" wide × 18" deep | 0.56 yd³ | 2.78 yd³ | 5.56 yd³ |
| 12" wide × 24" deep | 0.74 yd³ | 3.70 yd³ | 7.41 yd³ |
| 18" wide × 24" deep | 1.11 yd³ | 5.56 yd³ | 11.11 yd³ |
Trench volume before subtracting the pipe's displaced volume; the calculator's result will be slightly lower when pipe is included.
Frequently Asked Questions
How much gravel do I need for a French drain?+
Use the formula: Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards. A 50 ft trench at 12" wide × 18" deep has 2.78 yd³ of trench volume; the calculator subtracts the pipe's small displaced volume, showing about 2.62 yd³ (~3.7 tons) with 4" pipe. A 100 ft trench at the same size shows about 5.23 yd³ (~7.3 tons). Consider ~10% extra as a planning allowance for irregular trench walls and estimating uncertainty; it is not included in the result.
What size trench do I need for a French drain?+
It depends on the project. Trench width depends on pipe size, aggregate volume and the drainage problem, and 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 drainage systems can be deeper. PNNL's residential detail uses a trench at least about 22 inches deep and 14 inches wide for a 4-inch pipe — one installation detail, not a universal standard. The calculator's 12" × 18" default is a planning starting point.
What type of gravel is best for a French drain?+
Use clean, free-draining rock with minimal fines — avoid soil, crushed fines or dense-graded road base around the pipe. PNNL's residential detail uses clean rock roughly ¾–2 inches in size, but that isn't the only acceptable size for every drainage product. Use permeable filter fabric/geotextile intended for drainage to separate surrounding soil from the aggregate.
Do I need pipe in a French drain?+
Often, yes — PNNL describes a French drain as perforated pipe surrounded by rock and filter fabric. A 4" perforated pipe is a common residential starting point, and NDS says it is likely adequate for many residential applications, but heavy saturation or larger collection areas can require larger pipe. Drain capacity depends on pipe diameter, slope, inlet conditions, soil, aggregate and the amount of water reaching the system, and YardCalc estimates materials and geometry rather than performing stormwater hydraulic design.
How much does a French drain cost?+
At the calculator's current unit-price assumptions, materials for a 50 ft, 12" × 18" French drain with 4" pipe come to about $181–$382 (gravel + fabric + pipe); use the calculator for your dimensions. Professional installation costs $20–$50 per linear foot, or $1,000–$2,500 for a 50 ft drain — mostly labor. Permits may be required if the drain discharges to a roadway or waterway.
Can a French drain fix a wet basement?+
Not by itself. 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, and digging next to an existing foundation is best left to a qualified professional.
Common French Drain Applications
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Not sure of your trench run length? Measure it on the map →
What You'll Need
Beyond gravel, a French drain needs a few key supplies to work properly long-term.
4" Perforated Drain Pipe
Channels water out of the trench. Use 4" for standard residential drains, 6" for heavy runoff or downspouts.
Drain Pipe Filter Sock
Slips over the pipe to help keep sediment out of the pipe openings. It does not by itself replace filter fabric around the aggregate — follow your drainage system's detail.
Landscape Filter Fabric
Lines the trench walls to keep soil out of the gravel while still allowing water through freely.
Landscape Fabric Staples
Pins the filter fabric to the trench walls so it stays put when you add gravel. A rubber mallet helps a lot in firm or rocky soil — saves your hands considerably.
Want a professional to handle this?
French drain installation is labor-intensive digging work. Compare quotes from local drainage contractors to get an accurate picture of total project cost and timeline.
Get Local Quotes →Before You Order
- ☐Have underground utilities located before any trenching (811 in the US) and check local excavation requirements
- ☐Identify your outlet before digging — the pipe needs positive fall to a suitable discharge point, and local stormwater rules may apply
- ☐Check that your planned run has positive fall to the outlet — about 1% is a useful planning target, not a universal minimum
- ☐Plan permeable filter fabric/geotextile intended for drainage around the aggregate zone, and follow your drainage product's detail if you use a pipe sock
- ☐Plan cleanout or inspection access if your drainage design calls for it
Ordering in bulk?
French drains use clean, free-draining rock with minimal fines — for example #67 limestone, available for bulk delivery. Compare local pricing before you order at AggregateMarkets.
Prices and availability vary by location. Use code MEADOWLARK for 5% off your first bulk order.