Free estimate — verify against local code before building
French Drain Calculator
Estimate gravel, perforated pipe, geotextile, excavation, and slope fall for an entered drain trench.
What this calculator includes
Turn an entered trench section into a checkable material takeoff. The calculator subtracts the pipe cylinder from the gravel envelope, rounds pipe to stock lengths and aggregate to your supplier's order increment, and shows the total fall created by the entered slope.
How to use this french drain calculator
- 01
Measure the trench
Enter trench length, clear width, total excavation depth, and the depth of the gravel envelope around the pipe.
- 02
Match pipe and aggregate
Use the actual outside pipe diameter and stock length. Replace the planning gravel density with the supplier's tons-per-yard conversion.
- 03
Set fabric and slope
Choose whether fabric wraps the gravel envelope, enter roll coverage and overlap allowance, and enter the designed slope to see total fall.
- 04
Confirm the complete drainage path
Verify inlet, filtration, pipe elevation, legal outlet, overflow route, utilities, and downstream impacts with a qualified local professional before excavation.
Worked example
Example: 60-foot drain with a 4-inch pipe
A 60-foot trench that is 12 inches wide with a 12-inch gravel envelope and 4-inch pipe needs about 2.03 cubic yards of gravel, which rounds to 3 tons at 1.4 tons per yard. A 1% slope creates 7.2 inches of fall and just over 60 feet of sloped pipe, so 10-foot stock rounds to 7 pieces; a 10% fabric allowance produces 264 square feet, or one 300-square-foot roll.
Practical buying and overage guidance
Confirm washed aggregate size, pipe stiffness and fittings, fabric type, roll coverage, delivery minimums, and outlet parts as one system. Pipe stock rounding does not include couplings, cleanouts, catch basins, adapters, or outlet protection.
Continue the project
Upgrade Roof Drainage
Plan roof area, gutter runs, downspouts, fittings, extensions, drainage stone, grading, excavation, supplies, and contractor pricing.
Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- USDA NRCS National Engineering Handbook (opens in a new tab)
Engineering reference for site drainage, soil, and outlet considerations; it is not replaced by this quantity tool.
- EPA - Soak Up the Rain (opens in a new tab)
Stormwater planning resources emphasizing site conditions and responsible runoff management.
Frequently asked questions
Does French-drain gravel volume subtract the pipe?
Yes. The calculator finds the rectangular gravel envelope and subtracts the cylindrical pipe volume over the full trench length before converting to yards and tons.
What gravel density does the calculator use?
The default is only a planning conversion in tons per cubic yard. Aggregate size, moisture, and supplier gradation change delivered weight, so enter the quarry's conversion for the final order.
How is drainage fabric area calculated?
Fabric is treated as a full wrap around the gravel cross-section: bottom, two sides, and top, following the sloped drain length. The entered overlap and cut allowance is then added before rounding to whole rolls.
How much fall does a 1% slope create?
One percent means one foot of fall per 100 horizontal feet, or 0.12 inch per foot. The calculator multiplies entered length by slope percent and shows the total in feet and inches.
Will a French drain solve every wet-yard or foundation problem?
No. Soil, groundwater, grading, foundation details, pipe capacity, maintenance access, and a lawful outlet are site-specific. Diagnose the water source and obtain qualified drainage advice before building.