French Drain Calculator
Estimate trench volume, drainage gravel weight, perforated pipe length, fabric rolls and material cost for a straight French drain.
Default project preview
2.75 yd³
Perforated pipe
66 ft
Fabric rolls
1
Fabric area
264 ft²
Trench volume
3.33 yd³
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Drain length
Enter this project assumption in ft. The current imperial default is 60 ft.
Trench width
Enter this project assumption in in. The current imperial default is 12 in.
Trench depth
Enter this project assumption in in. The current imperial default is 18 in.
Gravel-filled portion
Enter this project assumption in %. The current imperial default is 75 %. Minimum: 10. Maximum: 100.
Extra allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Gravel density
Enter this project assumption in lb/ft³. The current imperial default is 100 lb/ft³.
Fabric roll length
Enter this project assumption in ft. The current imperial default is 100 ft.
Gravel price per ton or tonne
Enter this project assumption in a unitless value. The current imperial default is 50. Minimum: 0.
Pipe price per length unit
Enter this project assumption in per ft. The current imperial default is 2.5 per ft. Minimum: 0.
How to read the outputs
Perforated pipe
allowance included. Default preview: 66 ft.
Fabric rolls
100 ft roll length. Default preview: 1.
Fabric area
bottom and both sides. Default preview: 264 ft².
Trench volume
full excavation. Default preview: 3.33 yd³.
Gravel weight
bulk density estimate. Default preview: 3.71 tons.
Material cost
gravel and pipe. Default preview: $350.63.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Trench volume = length × width × depth. Gravel volume applies the filled percentage and allowance. Pipe length applies the same allowance; fabric width covers the bottom and both trench sides.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate trench excavation, drainage stone, pipe and fabric for a French drain from actual length and cross-section. It does not determine whether a French drain will solve the site's water problem.
Calculation steps
- 1Define trench length, width and depth.
- 2Separate bedding, side-fill and cover zones.
- 3Deduct pipe displacement where appropriate.
- 4Round stone, pipe and fabric to supplier units.
Worked numerical example
A 100 ft trench 18 in wide and 24 in deep with a 4 in pipe and 4 in bedding may require roughly 11.1 yd³ of gross trench material before deducting pipe and accounting for stone zones.
Purchasing and rounding
Pipe is rounded to full sections or coils, stone to truck or bulk units, and fabric to full rolls with overlap. Fittings and outlet components are separate.
Interpret the result
The quantity estimate describes a chosen section. Drain performance depends on outlet elevation, soil, slope and source of water.
Page-specific assumptions
- Uniform trench section.
- Pipe size and outlet are already selected.
- Stone voids are represented through bulk volume.
- Fabric layout is entered consistently.
Common mistakes
- Installing without a positive outlet.
- Using trench volume as stone volume without deductions.
- Ignoring fabric overlaps.
- Treating the calculator as hydraulic design.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
French Drain Calculator questions
Should a French drain have a continuous outlet?+
Most systems need a reliable gravity outlet or engineered discharge point. A trench without an outlet can store rather than remove water.
How much slope should the pipe have?+
Required slope depends on the design and site. Use the drainage-slope calculator as a planning aid and verify the final grade.
Why is only part of the trench filled with gravel?+
Some installations include soil cover or other layers above the drainage aggregate. Enter the gravel-filled percentage for the detail being used.