How much gravel, pipe, and fabric do you need for a French drain? Calculate exact materials and costs for your drainage project in seconds.
10% recommended for gravel settling and irregular trench walls
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Last updated March 31, 2026 by our expert review team
Small landscape areas
Under 300 sq ft
Standard residential
300-1000 sq ft
Large or heavy flow
Over 1000 sq ft
Note: 4-inch perforated pipe is standard for most residential applications. Upgrade to 6-inch for areas with heavy water flow or when draining large surfaces.
Ruth Wairimu
Landscape Architect
Landscape architect with 9+ years of experience, AAK member, IFLA climate activist, and founder of Bloomwell.
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Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated March 2026
How we verify our calculatorsUse 4-inch perforated pipe for standard residential drains. NDS and ADS are the leading pipe manufacturers. It handles up to 1,000 sq ft of drainage area and is the most available size at home centers.
Maintain a minimum 1% slope (1/8 inch per foot) from high point to outlet. Use a string line or laser level to verify grade before laying pipe.
Always use 3/4-inch clean washed gravel (#57 stone). Avoid crushed limestone or recycled concrete, which compact over time and reduce flow rates by up to 60%.
Wrap the entire trench with non-woven geotextile fabric (4 oz/sq yd minimum) before adding gravel. Mirafi 140N is the industry standard. Overlap seams by at least 6 inches.
Terminate at a daylight outlet whenever possible. Position the outlet at least 10 feet from the foundation with the discharge area sloping away.
Install a cleanout fitting every 50 feet and at every 90-degree turn so you can flush sediment with a garden hose during annual maintenance.
Need drainage stone? Our crushed stone calculator estimates #57 stone tons and costs for drain backfill.
Our French drain calculator determines the exact amount of gravel, perforated pipe, and landscape fabric needed for your drainage project. Use our pea gravel calculator if you need decorative drainage gravel, or our excavation calculator to estimate dirt removal for the trench. It starts by computing the trench volume in cubic feet (length × width × depth), then subtracts pipe displacement (the space the perforated pipe occupies inside the trench) so you only purchase the gravel you actually need. A 4-inch pipe displaces roughly 10% of trench volume, while a 6-inch pipe displaces about 15%.
The fabric estimate uses the wrap method: length × (width + 2 × depth). This accounts for lining the trench bottom and both vertical walls with a single continuous sheet of geotextile, which is the industry-standard installation technique. Wrapping prevents soil particles from migrating into the gravel envelope, which is the number-one cause of French drain failure.
The waste factor slider (default 10%) compensates for irregular trench walls, gravel settling after backfill, and minor measurement variations. For rocky or root-heavy soil where trench walls tend to crumble, increase waste to 15-20%. For clean, sandy soil with straight trench walls, 5-8% is usually sufficient.
Gravel Formula
Volume = (Length × Width × Depth) × (1 - Pipe Displacement %) × (1 + Waste %)
Convert to cubic yards: Volume ÷ 27 (there are 27 cubic feet in one cubic yard)
Fabric Formula (Wrap Method)
Sq Ft = Length × (Width + 2 × Depth)
Covers trench bottom + both vertical walls with overlap for folding over the top
How much does a French drain cost per linear foot?
DIY costs $10 to $16/LF for materials. Professional installation runs $25 to $60/LF depending on depth and soil conditions.
How deep should I dig a French drain?
Dig 18 to 24 inches for most residential applications. Minimum functional depth is 12 inches. For foundation drains, dig to the footing bottom.
Interior vs exterior French drain?
Exterior drains intercept groundwater before it reaches your foundation. Interior drains run along the basement slab edge and channel water to a sump pit.
Do I need a sump pump?
Only if the drain cannot reach a daylight outlet (no downhill slope). Interior basement drains almost always need a sump pump.
How do I maintain a French drain?
Flush cleanout ports yearly with a garden hose. Inspect the outlet for blockages from debris, roots, or animal nests.
Can I use a French drain in clay soil?
Yes, but use a wider trench (at least 12 inches) and always wrap with geotextile fabric. Clay fines clog unwrapped drains within 2 to 3 years.
Should pipe holes face up or down?
Holes down (4 and 8 o'clock positions) for groundwater collection. Holes up only when capturing surface water percolating through gravel.
What size pipe do I need?
4-inch is standard for most residential drains (up to 1,000 sq ft). Use 3-inch for short runs under 300 sq ft, 6-inch for large lots.
30 ft long, 10" wide, 18" deep | 4" pipe | ~$180 in materials
Tip: A 30-foot French drain with a daylight outlet at the property line solves persistent standing water after rain.
120 ft long, 12" wide, 24" deep | 4" pipe | ~$850 in materials
Tip: Two cleanout risers at corners allow annual flushing. Tie into an existing downhill storm drain easement.
50 ft long, 8" wide, 14" deep | 3" pipe | ~$110 in materials
Tip: The narrower trench and smaller pipe keep costs low while stopping erosion along the driveway edge.
Mark drain path with spray paint. Dig trench 12-24" deep with 1-2% slope.
Line trench bottom and sides with landscape fabric, leaving extra for wrap.
Add 2-3" of gravel base. Ensure proper slope with level or laser.
Place perforated pipe holes-down. Connect sections with couplers.
Cover pipe with gravel to 2-4" below ground level. Maintain slope.
Fold fabric over gravel. Top with soil or decorative stone.
Skipping the Geotextile Fabric
Without fabric, soil fines migrate into gravel within 1 to 3 years and choke the drain. Use non-woven fabric rated at 4 oz/sq yd minimum.
Insufficient or Zero Slope
Water needs gravity to move. Maintain at least 1% slope (1/8 inch per foot). Verify with a string line before backfilling.
Using the Wrong Gravel
Crushed limestone and recycled concrete compact over time. Specify 3/4-inch clean washed #57 stone for proper void space.
No Outlet or Dead-End Drains
The drain must terminate at a daylight outlet or sump pit. Dead-end drains fill up and overflow back toward the problem area.
Trench Too Narrow for the Pipe
Allow at least 2 inches of gravel on every side of the pipe. For 4-inch pipe, minimum trench width is 8 inches; 12 inches is recommended.
Backfilling Soil Directly on Gravel
Fold fabric flaps over the gravel before adding soil. Exposed gravel lets fines wash into the drain with every rain.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.