French Drain Calculator
Estimate washed drain stone, perforated pipe, filter fabric, and slope drop for a landscape French drain trench.
Trench Dimensions
Ready to calculate
Enter your trench dimensions
Last updated June 2, 2026 by our expert review team
Pipe Size Selection
3" pipe
Compact landscape runs
Confirm flow capacity
4" pipe
Common residential start
Check outlet and site
6" pipe
Higher-flow projects
Get site-specific advice
Note: Pipe size depends on the water source, run length, slope, soil, and outlet. A 4-inch perforated pipe is a common residential starting point, not a universal answer.
Expert Contributors
Ruth Wairimu
Landscape Architect
Landscape architect with 9+ years of experience, AAK member, IFLA climate activist, and founder of Bloomwell.
See full profileHawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
How we verify our calculatorsUse Open Drain Stone, Then Slope to an Outlet
A French drain works because water can move through open stone into a perforated pipe and toward a real outlet. Dense base material and compacting fines defeat that purpose.
Pro Tips
Plan the outlet first. A gravel trench with no safe discharge point can move the problem instead of solving it.
Use clean washed drainage stone with open void space. Avoid crusher run, stone dust, and dense base material around the pipe.
A 1% slope is a useful planning default when gravity drainage is possible. Confirm the available drop from the high point to the outlet before digging.
A 4-inch perforated pipe is a common residential starting point. Use an installation guide such as NDS French drain guidance, then match the pipe and outlet to your site.
Filter fabric selection depends on the soil and drainage design. Keep soil fines out of the stone envelope and follow the product instructions.
Call for utility marking before excavation. Foundation, retaining-wall, and high-flow drainage problems may need site-specific professional advice.
Need drainage stone? Our crushed stone calculator estimates #57 stone tons and costs for drain backfill.
How the Calculator Works
Enter the trench length, width, and depth. The calculator finds the trench volume, subtracts the actual circular pipe volume, and adds your ordering allowance. Use cubic yards to order clean washed drainage stone.
The filter-fabric estimate covers the trench bottom, both side walls, and a simple top overlap. The slope field turns your percentage into a total drop, so you can check whether the planned outlet is realistic.
Use our excavation calculator when you also need a spoil-removal estimate. For decorative surface stone outside the trench, the pea gravel calculator is a better fit.
Gravel Formula
Stone = (Trench Volume - Circular Pipe Volume) × (1 + Allowance)
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 + 1 ft overlap)
Covers trench bottom, both side walls, and a simple top overlap.
Quick Reference
- Stone type
- Clean washed
- Avoid
- Compacting fines
- Planning slope
- 1% (1.2"/10')
- Outlet
- Plan before digging
- Allowance
- 10% default
- Pipe size
- 4" standard
Frequently Asked Questions About French Drains
How much stone do I need for a French drain?
Measure the trench, subtract the pipe volume, and add an ordering allowance. This calculator leads with cubic yards of washed drainage stone.
How deep should I dig a French drain?
Depth depends on the water source, outlet, frost conditions, nearby structures, and local requirements. Use 18 inches as a planning default, then confirm the site design.
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?
A gravity outlet is simplest when the site allows it. If water cannot discharge safely downhill, get site-specific advice before choosing a sump or another outlet.
How do I maintain a French drain?
Keep the outlet clear and inspect the system after heavy rain. Add cleanouts when the installation plan calls for access.
Can I use a French drain in clay soil?
Clay makes filtration and outlet planning more important. Use filter fabric suited to the soil and ask for site-specific help when drainage is persistent or near a structure.
Should pipe holes face up or down?
Follow the pipe manufacturer's installation instructions. Pipe orientation depends on the product and drainage design.
What size pipe do I need?
A 4-inch perforated pipe is a common residential starting point. Capacity depends on the water source, run length, slope, and outlet.
French Drain Estimates for Common Projects
Backyard Soggy Spot
30 ft long, 10" wide, 18" deep | 4" pipe | about 1.4 cu yd stone
Tip: Plan the outlet before digging. Confirm the route after utility marking.
Foundation Perimeter Drain
120 ft long, 12" wide, 24" deep | 4" pipe | about 9.4 cu yd stone
Tip: Drainage beside a foundation deserves site-specific design and a verified discharge plan.
Driveway Edge Drain
50 ft long, 8" wide, 14" deep | 3" pipe | about 1.5 cu yd stone
Tip: Confirm that the trench, pipe, and outlet are sized for the water source.
French Drain Installation Steps
Plan and Mark
Choose the outlet, mark utilities, and confirm the available drop before digging.
Excavate the Trench
Dig the planned width and depth while maintaining the design slope.
Lay Filter Fabric
Line the trench with the fabric selected for your soil and leave enough for overlap.
Place Stone and Pipe
Add washed stone and install the perforated pipe according to its instructions.
Fill with Washed Stone
Keep compacting fines out of the drainage envelope.
Wrap and Finish
Fold the fabric over the stone and finish the surface for the site.
French Drain Mistakes That Cost You Money
Digging before planning the outlet
Confirm where water can discharge safely and whether gravity provides enough drop.
Using compacting base material
Crusher run and stone dust reduce void space. Use clean washed drainage stone around the pipe.
Ignoring soil filtration
Choose filter fabric for the soil and drainage design so fines stay out of the open stone envelope.
Guessing the pipe size
Pipe capacity depends on the water source, length, slope, and outlet. Four-inch pipe is a starting point, not a rule.
Forgetting utility marking
Call for utility marking before excavation and adjust the route when needed.
Treating foundation drainage as a simple trench
Drainage beside a structure can affect the building and neighboring property. Get site-specific advice.
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.