French Drain Materials Calculator

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Drain profile:

Drain run length:

Trench width:

Trench depth:

Soil or surface cover:

Pipe mode:

Pipe diameter:

Target slope:

Adjust with slider

Available fall to outlet:

Outlet type:

Fabric wrap:

Fabric overlap:

Fabric roll width:

Fabric allowance:

Drain rock:

Stone bulk density:

kg/m³

Stone allowance:

Adjust with slider

Pipe section length:

Pipe extra allowance:

Outlet tail length:

Cleanout spacing:

Stone order unit:

Stone order increment:

Optional USD prices add a material-only subtotal. Zero leaves every quantity, check and chart value unchanged.

Stone price:

$/ {{ orderUnitLabel }}

Pipe price:

$/ m

Fabric price:

$/ m²

Fitting allowance:

French drain material takeoff with order quantity, cross-check, optional cost and estimating basis.
Material	Order	Cross-check	Estimated cost	Basis	Copy
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A French drain gives subsurface water a more open route through soil. Clean stone creates connected voids, geotextile helps keep fine soil out of those voids, and perforated pipe can carry collected water toward an outlet. The trench still needs a lower, legal place to discharge. Without one, it becomes underground storage that can fill during sustained rain.

Site diagnosis comes before material ordering. Persistent wet ground may come from uphill seepage, grading, downspouts, irrigation leaks, a high water table, or runoff concentrated by paving. A buried drain can intercept or redirect some of that water, but it cannot fix an outlet that is too high, a dry well with too little storage, or a foundation problem that needs engineered waterproofing.

The stone quantity is controlled by the three-dimensional envelope below any soil or surface cover. Run length, trench width, and stone depth make a rectangular volume. A perforated pipe displaces part of that volume, then a stone allowance covers ordering loss and field variation. Deeper trenches grow the order quickly because the added depth extends along the entire run.

French drain planning terms and material consequences
Term	Meaning	Why it matters
Stone depth	Trench depth minus soil or surface cover	Sets stone volume and fabric wrap depth
Fall	Vertical drop from the high end to the outlet	Must cover the target slope along the whole run
Fabric strip width	Bottom width, both sides, and any top overlap	A roll narrower than this strip requires a different detail or seams
Bulk density	Supplier mass per loose cubic metre of stone	Converts geometric volume into tonnes or short tons

Fabric quantity depends on the cross-section, not only the drain length. A full wrap covers the bottom, both sides, and a top overlap. A liner covers the bottom and sides without the top fold. End caps and a trimming allowance add more area. The roll width then determines how much roll length is needed and whether one continuous strip can span the section.

Pipe diameter affects stone displacement and clearance, but it does not size hydraulic capacity here. Flow capacity also depends on pipe perforations, stone gradation, soil permeability, inflow, bends, outlet conditions, and maintenance. Cleanouts and an accessible, protected outlet can matter as much as the initial material quantity.

Excavation adds risks that a takeoff cannot resolve. Buried utilities must be marked, trench safety must match depth and soil conditions, and foundation, retaining-wall, steep-slope, repeated-flooding, or storm-connection work may require a qualified professional or local approval.

How to Use This Tool:

Build the cross-section first, then verify the fall and outlet before relying on the material order.

Choose a Drain profile as an editable starting point, then enter the measured run length, trench width, trench depth, and soil or surface cover.
Select perforated pipe or a gravel-only drain. When pipe is included, enter its diameter and confirm that the resulting side and top stone clearances fit the project detail.
Enter the target slope and measured Available fall to outlet, then choose the outlet type. Stop if the fall is short or no lower discharge path is confirmed.
Choose the fabric wrap, roll width, and clean drain stone. Replace preset bulk density with the supplier's loose density when ordering by weight.
Open Advanced for allowances, pipe-section length, cleanout spacing, order increments, and prices. Read installation checks before using the rounded stone order.

Interpreting Results:

The rounded stone order is expressed in the selected supplier unit. Cross-check it against cubic metres and metric tonnes because density-based weight and geometric volume answer different ordering questions. A supplier's loose bulk density should replace the preset before a weight order is placed.

Fall covered means available end-to-end fall is at least the calculated requirement. Field levels must still confirm a continuous grade without sags.
Fabric too narrow means the selected roll width is less than the calculated strip width. The area total alone does not solve that width mismatch.
Pipe clearance review means either side clearance or stone above the pipe is below the 50 mm planning check.
Outlet warnings identify capacity, permit, freeze, erosion, maintenance, or missing-path questions; they do not approve the discharge.

Technical Details:

The takeoff converts all length inputs to metres, calculates the stone-filled cross-section, subtracts cylindrical pipe displacement when pipe is included, and then adds material allowances. Separate rules turn that geometry into supplier units, pipe sections, fabric roll length, status checks, and optional cost.

Formula Core

Stone depth is the part of the trench below surface cover. The gross envelope is rectangular, while pipe displacement is cylindrical.

d_{s} = d_{t} - d_{c}, V_{g} = L W d_{s}, V_{p} = π {(\frac{D}{2})}^{2} L

The stone order subtracts pipe displacement, prevents a negative base volume, and then adds the entered stone allowance.

V_{o} = \max (0, V_{g} - V_{p}) (1 + \frac{w}{100})

Required fall is run length multiplied by slope as a percentage. Fabric area uses the chosen strip width, two end caps, and a fabric allowance.

F = L \frac{s}{100}, B = W + 2 d_{s} + o, A_{f} = [L B + 2 W d_{s}] (1 + \frac{a}{100})

French drain formula symbols and units
Symbol	Meaning	Unit
`L`	Drain run length	m
`W`	Trench width	m
`d_s`	Stone depth below surface cover	m
`D`	Pipe diameter, or zero for gravel-only mode	m
`w`	Stone allowance	%
`s`	Target slope	%
`o`	Top overlap for a full fabric wrap	m
`a`	Fabric allowance	%

A liner-only strip omits top overlap; no-fabric mode returns zero strip width and area. Fabric roll length is area divided by entered roll width. Pipe length equals the run plus the pipe-extra percentage and outlet tail, then rounds up to whole sections. Couplers are one fewer than sections, never below zero. Stone volume converts to mass with entered bulk density, to cubic yards with 1 yd³ = 0.764554857984 m³, and to short tons with 1 short ton = 907.18474 kg. A positive order increment rounds the selected supplier quantity upward.

Rule Core

French drain status rules and exact boundaries
Check	Passing or warning rule	Meaning
Fall	Available fall ≥ required fall	Covered at slope ≥ 1%; low slope when 0% < slope < 1%; flat at 0%; otherwise short
Pipe clearance	Side clearance ≥ 0.05 m and top stone cover ≥ 0.05 m	Both boundaries are inclusive; otherwise review
Fabric width	Roll width ≥ calculated strip width	Equality fits; a narrower roll is flagged
Outlet	Depends on selected outlet type	Daylight ready; pop-up review; dry well capacity check; storm connection permit check; no outlet blocked

The fall comparison and whole-unit rounding use a 10⁻¹² tolerance to prevent floating-point noise at an exact boundary. Cost is the sum of rounded stone order times stone price, pipe length times pipe price, fabric area times fabric price, and the entered fitting allowance.

Limitations and Safety Notes:

This is a material takeoff and planning check, not hydraulic, geotechnical, structural, waterproofing, or stormwater design.

Stone density presets are planning values. Supplier gradation, moisture, compaction, and delivered bulk density can change a weight order.
End elevations do not prove a continuous slope; verify the trench bottom with field levels and correct low spots.
Outlet capacity, infiltration, freeze behavior, erosion, property-line effects, permits, and maintenance need separate review.
Arrange utility marking and appropriate trench protection before excavation.

Worked Examples:

A fabric width mismatch in a 15 m yard drain

A 15 m run, 300 mm trench width, 450 mm trench depth, and 50 mm surface cover leave 400 mm of stone depth. With 100 mm pipe and a 12% stone allowance, the estimate is about 1.884 m³ or 3.014 metric tonnes at 1,600 kg/m³. A 1% slope needs 150 mm of fall, so 180 mm available leaves a 30 mm margin. The full wrap needs a 1.25 m fabric strip, which makes a 1.2 m roll too narrow even though the calculated fabric area is 20.889 m². That warning should be resolved before ordering the roll.

References:

Installing and Maintaining a French Drain, LSU AgCenter, Publication 3879, 2023.
Environmentally Sensitive Road Maintenance Practices for Dirt and Gravel Roads, U.S. Forest Service.
Appendix C: General Tables of Units of Measurement, National Institute of Standards and Technology, 2026.