French Drain Materials Calculator

Measurement system:

Use metric defaults, or switch to imperial before entering field measurements.

Drain profile:

Pick the closest job type, then adjust the measurements below.

Drain run length:

Enter the total trench path, including bends.

Trench width:

Use the average clear trench width.

Trench depth:

Depth controls gravel volume, fabric wrap, and pipe cover checks.

Soil or surface cover:

Enter any top cover above the wrapped stone envelope.

Pipe mode:

Choose whether this material takeoff includes pipe.

Pipe diameter:

Used for pipe volume displacement and clearance checks.

Target slope:

Set the planned grade from high end to outlet.

Slope slider {{ formatNumber(slopePercentValue, 1) }}%

Available fall to outlet:

Used to flag whether the planned slope can drain by gravity.

Outlet type:

Choose how the drain will discharge.

Fabric wrap:

Full wrap is the normal anti-clog takeoff for a stone-and-pipe French drain.

Drain rock:

Pick the closest aggregate and confirm density with the supplier when ordering by weight.

Waste and rounding allowance:

Keep a modest allowance unless the trench is very irregular.

Allowance slider {{ formatNumber(wastePercentValue, 0) }}%

Stone density:

Used to convert gravel volume to tonnes and short tons.

kg/m3

Fabric overlap:

Only applied to full wrap mode.

Fabric roll width:

Set the roll width available at the supplier.

Fabric allowance:

Leave at 0 for exact calculated area.

Pipe section length:

Set the straight pipe length sold locally.

Pipe extra allowance:

Applied before outlet tail length is added.

Outlet tail length:

Set to 0 when the measured drain run already includes this length.

Cleanout spacing:

One intermediate cleanout is added for each interval beyond the outlet ends.

Stone order unit:

Choose the supplier quote unit for rounded stone orders.

Stone order increment:

Use 0 to keep the exact material estimate.

Stone price:

Enter 0 to omit stone cost.

$ / {{ orderUnitShortLabel }}

Pipe price:

Enter 0 to omit pipe cost.

$ / m

Fabric price:

Enter 0 to omit fabric cost.

$ / m2

Fitting allowance:

Enter 0 when fittings are priced separately.

Material	Order quantity	Cross-check	Buying note	Copy
{{ row.material }}	{{ row.quantity }}	{{ row.crossCheck }}	{{ row.note }}

Check	Status	Detail	Copy
{{ row.check }}	{{ row.status }}	{{ row.detail }}

Scenario	Stone depth	Stone volume	Fabric area	Material note	Copy
{{ row.scenario }}	{{ row.stoneDepth }}	{{ row.stoneVolume }}	{{ row.fabricArea }}	{{ row.note }}

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A wet strip beside a patio, a soft low spot in a lawn, or damp soil against a foundation is usually a routing problem before it is a materials problem. Water needs a path that is lower, open enough to carry flow, and protected from the fine soil that can clog that path after the trench is buried.

A French drain supplies that buried path with clean stone, optional perforated pipe, and geotextile fabric. The stone creates open voids, the pipe gives collected water a steadier route toward the outlet, and the fabric reduces soil migration into the stone. The assembly works only when the route has enough fall to discharge somewhere acceptable.

Material estimating depends on the physical section of the trench, not just the visible distance across the yard. Run length controls pipe and fabric length. Trench width and stone depth control gravel volume. Surface cover reduces the stone section. A perforated pipe displaces some rock while adding pipe sections, couplers, outlet length, and cleanout planning.

French drain planning terms that affect material quantities
Planning term	Plain meaning	Why it changes the order
Stone envelope	The rock-filled part of the trench below any soil, turf, mulch, or surface cover.	It sets the gravel volume and the fabric strip width.
Fall	The vertical drop from the high end of the drain to the outlet end.	Too little fall can leave a correct material list attached to a drain that will not empty by gravity.
Outlet	The legal, lower discharge point where collected water can leave the problem area.	A dry well, pop-up emitter, storm connection, or daylight outlet can add review, capacity, or permit checks.
Filter fabric	A geotextile separator around the stone or along the trench bottom and sides.	The wrap choice changes fabric width, fabric area, roll length, and overlap waste.

French drain cross-section showing fabric wrap, clean stone, pipe, trench width, stone depth, and slope to outlet

French drains are commonly used to intercept uphill seepage, dry a soggy yard edge, carry water away from a driveway or patio, or help a foundation perimeter drain reach a safe discharge point. Those uses share the same practical requirement: the drain must move water toward somewhere lower. Without that destination, the trench becomes temporary underground storage that can fill during a long storm.

Small measurement errors can change the order more than expected. An extra few inches of depth along a long run can add a large amount of stone. A narrow fabric roll can leave the top of the stone exposed. A pipe in a tight trench can lack enough side clearance or top cover. Clean washed stone matters because fines and mud can fill the voids that were supposed to carry water.

A takeoff is useful before buying materials, but it is not a drainage design approval. Soil texture, groundwater, frost depth, utility locations, foundation details, erosion risk, and local discharge rules still decide whether the route is suitable and safe to dig.

How to Use This Tool:

Start with the trench you expect to dig, then use the checks to see whether the material count still makes sense for the outlet, slope, fabric, and pipe clearance.

Choose Measurement system before replacing defaults so length, width, depth, pipe diameter, fabric, and ordering units match the measurements you have on site.
Pick the closest Drain profile to load a starting run length, trench size, pipe size, slope, outlet, fabric, stone, and waste allowance. Use Custom drain plan when the defaults do not match the project.
Enter Drain run length, Trench width, Trench depth, and Soil or surface cover. If cover equals or exceeds trench depth, the tool asks for a valid stone section before it estimates materials.
The cover value is capped to the trench depth, but a zero stone section blocks a useful gravel takeoff.
Set Pipe mode. With Perforated pipe included, pipe diameter affects rock displacement, clearance, pipe sections, couplers, cleanouts, and outlet fittings. With Gravel-only curtain drain, pipe quantities and pipe displacement are omitted.
Use Target slope, Available fall to outlet, and Outlet type to check whether the run can drain by gravity and whether the discharge path needs review, permitting, or a separate capacity check.
A slope below 1% is marked low, and an available fall below the required fall is marked short even when material quantities can still be calculated.
Choose Fabric wrap, Drain rock, and Waste and rounding allowance. Open Advanced for fabric overlap, fabric roll width, stone density, pipe section length, cleanout spacing, supplier order increments, and optional material prices.
Read Material Takeoff first, then use Slope Outlet Checks, Trench Scenarios, Gravel Depth Curve, and JSON to verify the assumptions before ordering.

Interpreting Results:

Drain rock is the main ordering number because it usually controls delivery volume, tonnage, and price. Compare the rounded order with the exact volume or weight cross-check before calling a supplier, especially when the quote uses cubic yards, cubic metres, short tons, or metric tonnes.

Required fall and Available fall decide whether the selected grade can work by gravity. A complete stone, pipe, and fabric list is not ready to buy when the outlet fall is short or the outlet status says blocked, capacity check, permit check, or review.

French drain result areas and interpretation checks
Result area	Use it for	Do not overread it as
Material Takeoff	Stone order, pipe sections, fabric area, fittings, excavation reference, and material-only subtotal.	A full contractor bid or proof that the wet area will drain.
Slope Outlet Checks	Gravity fall, outlet status, pipe clearance, fabric width, stone specification, and field safety prompts.	A permit approval, utility locate, hydraulic design, or legal discharge decision.
Trench Scenarios	A shallower/current/deeper comparison for stone volume, tonnage, fabric area, and pipe cover.	Permission to reduce depth below the route, grade, frost, or drainage requirement.
Gravel Depth Curve	A sensitivity check showing how stone depth changes rock volume and fabric area.	A replacement for measuring the trench depth and outlet elevation in the field.

Before buying, compare the estimate with field measurements taken along the full route, confirm the outlet elevation with a level, and ask the supplier for actual aggregate density and order increments. A modest width or depth change over a long trench can move the rock order more than the final rounding setting.

Technical Details:

French drain estimating is a net-volume calculation. The trench contributes a rectangular stone section after surface cover is removed, and perforated pipe subtracts a circular volume from that section when pipe is included. The remaining stone volume is then increased by the selected waste allowance before order rounding.

Fabric estimating is based on strip width. A liner follows the bottom and both sides of the stone envelope, while a full wrap adds overlap so the fabric can fold over the top. End caps and a fabric allowance cover the two open trench ends, folds, trimming, wrinkles, and seams.

Formula Core:

The core equations calculate stone depth, net stone volume, adjusted order basis, required fall, fabric strip width, and pipe order length.

\begin{array}{lcl} D_{stone} & = & max (0, D_{trench} - D_{cover}) \\ V_{stone} & = & max (0, L \times W \times D_{stone} - π \times {(d / 2)}^{2} \times L) \\ V_{order} & = & V_{stone} \times (1 + A / 100) \\ F_{required} & = & L \times S / 100 \\ W_{fabric} & = & W + 2 \times D_{stone} + O_{wrap} \\ P_{order} & = & L \times (1 + E / 100) + T \end{array}

Here, L is drain run length, W is trench width, D_stone is stone depth, d is pipe diameter, A is stone waste allowance, S is slope percent, O_wrap is full-wrap overlap, E is pipe extra percent, and T is outlet tail length. Gravel-only mode treats pipe diameter, pipe order length, couplers, and cleanouts as zero.

A 15 m run that is 0.30 m wide and 0.45 m deep with 0.05 m of cover has 0.40 m of stone depth. The gross stone section is 15 x 0.30 x 0.40, or 1.80 m3. A 0.10 m pipe displaces about 0.12 m3, leaving about 1.68 m3 before allowance. A 12% stone allowance raises the estimate to about 1.88 m3 before any supplier increment rounds the order upward.

Validation Boundaries:

French drain calculation boundaries and checks
Check	Boundary used	Why it matters
Required dimensions	Run length, trench width, and trench depth must be positive.	Zero or missing geometry cannot produce a material takeoff.
Stone depth	Cover depth must be less than trench depth.	A cover layer that consumes the trench leaves no rock section.
Target slope	Slope is limited to 0% to 5%; 1% or higher is marked typical.	Low slopes are more sensitive to settlement, excavation error, and flat spots.
Available fall	Available fall should meet or exceed required fall.	Rock and pipe cannot make a gravity drain work uphill.
Pipe clearance	About 0.05 m side clearance and 0.05 m stone above the pipe are checked.	The pipe needs stone around it, not only a pipe-sized slot.
Fabric roll width	Roll width is compared with the calculated strip width.	A narrow roll may require seams, a different wrap, or a wider fabric product.
Stone density	Custom density is constrained to 400 to 2800 kg/m3.	Density controls the tonnage estimate and material cost when buying by weight.

The recommended stone order is rounded only after the exact volume or weight is calculated. If the supplier sells in 0.5 cubic yard or 0.25 tonne increments, the order quantity moves upward to the next increment while the exact takeoff remains available as a cross-check.

Advanced Tips:

Use the supplier's actual stone density when ordering by weight. The default density is a planning estimate, and wet or unusually graded aggregate can change the tonnage cross-check.
Set Order increment to the supplier's minimum delivery step before reading the rounded stone order. Leave it at zero when you want the exact calculated amount.
Compare Trench Scenarios before reducing depth. A shallower trench can save stone while also reducing pipe cover and changing fabric width.
Use Gravel Depth Curve to find the depth range where rock volume rises sharply. That chart is useful when a few inches of extra excavation would trigger another delivery increment.
Enter fabric roll width and overlap before buying fabric. Full wrap can need a wider roll than a bottom-and-side liner, especially in deep trenches.
Add material prices only for a material subtotal. Delivery, equipment, spoil hauling, restoration, labor, taxes, permits, and design review are outside the subtotal.

Limitations and Privacy Notes:

The calculator estimates material quantities from trench geometry and user-entered assumptions. It does not size inflow rate, storm duration, pipe hydraulic capacity, soil infiltration, groundwater pressure, frost depth, retaining-wall loads, foundation waterproofing, sump performance, or dry-well storage.

Call utility-locating services before excavation and verify the outlet route before backfilling.
Foundation drains, retaining walls, repeated flooding, public storm connections, and steep slopes may need professional drainage or engineering review.
Calculations run from the values entered in the browser. If you share a configured page URL, check whether the URL includes project measurements you would rather keep private.

Worked Examples:

Wet lawn interceptor:

A 15 m yard run, 0.30 m trench width, 0.45 m trench depth, 0.05 m top cover, 100 mm perforated pipe, and 12% stone allowance produces about 1.88 m3 of drain rock. At 1600 kg/m3, the cross-check is about 3.0 metric tonnes before supplier rounding.

Short outlet fall:

A 20 m run at a 1% slope needs 0.20 m of vertical fall. If the measured outlet fall is only 0.12 m, the material takeoff can still be calculated, but the slope check marks the run short. The correction is a lower outlet, shorter run, revised route, or different drainage plan, not more stone.

Fabric roll too narrow:

A trench that is 0.35 m wide with 0.40 m of stone depth needs 1.15 m of fabric strip width before overlap. With a 0.15 m full-wrap overlap, the strip width becomes about 1.30 m. A 1.2 m roll is likely too narrow unless the wrap detail changes or seams are acceptable.

Gravel-only curtain drain:

When pipe is omitted, the same trench keeps the full stone envelope because no pipe volume is subtracted. Pipe sections, couplers, cleanouts, and pipe-related clearance checks are skipped, which can suit a simple seepage interceptor but is not the same as sizing a piped drain.

FAQ:

Does a French drain always need perforated pipe?

No. A gravel-only drain can collect low-flow seepage, but perforated pipe usually improves carrying capacity, provides a clearer route to the outlet, and makes longer runs easier to maintain.

Why does top cover reduce the gravel quantity?

Top cover is soil, turf, mulch, pavement bedding, or another surface layer above the stone envelope. It is not counted as drain rock, so it is subtracted from trench depth before stone volume is calculated.

Is washed stone different from ordinary gravel?

Washed drain stone has fewer fines that can plug voids and reduce flow. Angular stone often locks together better than rounded rock, but the best specification depends on local materials and the drainage detail.

Why is a legal outlet so important?

The drain must discharge somewhere lower and acceptable. If water exits where it can freeze, pond, erode soil, return to the same wet area, enter a neighbor's property, or connect illegally to storm infrastructure, the material takeoff is not enough.

Can the cost subtotal be used as a contractor quote?

No. The subtotal covers only the material prices entered for stone, pipe, fabric, and fitting allowance. Labor, equipment, spoil hauling, restoration, permits, taxes, delivery, and design work are separate.

Glossary:

French drain: A buried drainage trench filled with rock, with or without perforated pipe, that moves water toward an outlet.
Stone envelope: The drain-rock section inside the trench after top cover is excluded.
Perforated pipe: Drain pipe with slots or holes that collect water from the stone and carry it toward the outlet.
Filter fabric: Geotextile used to separate soil from drain rock while still allowing water through.
Daylight outlet: A discharge point where the pipe exits to open air at a lower elevation.
Required fall: The vertical drop needed to achieve the selected slope over the drain run.

References:

Installing and Maintaining a French Drain, LSU AgCenter.
Trail Construction and Maintenance Notebook: Trails in Wet Areas, USDA Forest Service.
Wetland Trail Design and Construction: Improving Drainage, USDA Forest Service.
2021 International Residential Code Section R405.1, International Code Council.