Drainage & Earthworks
French Drain Calculator
Calculates French drain materials
Updated May 13, 2026 · Live
What this tool does
Calculates French drain materials. Trench gravel, perforated pipe, fabric.
Estimates gravel volume, perforated pipe length, and filter fabric area for a French drain trench.
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Formula Used
How the french drain calculator works
Calculates French drain materials. Trench gravel, perforated pipe, fabric. The calculator takes your dimensions and supplier rates, applies a standard US construction formula, and returns a quantity with an indicative cost. Every figure is an estimate — site conditions always move the final number.
Typical US drainage and earthworks wastage
Excavated soil bulks up 15–35% when lifted, so dumpster capacity fills faster than the in-place volume suggests. Plan a 1.25× swell factor when sizing haul-off. Our defaults reflect common US trade allowances, and can be adjusted upwards for non-standard geometry or downwards where experience supports a lower figure.
What this tool does not do
It does not replace a professional quote, factor regional pricing, assess structural adequacy, or confirm building code compliance. Those remain the responsibility of a suitably qualified designer, engineer, or your local building official.
On-site considerations for french drain
Excavations over 4 ft deep need shoring, sloping, or benching per OSHA 1926 Subpart P. Call 811 (Dig Safely) at least 48–72 hours before any excavation to mark utilities.
Codes and compliance
Foul and storm drainage follows the IPC and local stormwater rules; many jurisdictions require permits for tie-ins to public sewer or storm drains. Dry wells (drywells) need a perc test to size correctly. When in doubt, file a pre-application question with your local building department — early clarity is cheaper than a corrective inspection.
Before you order
Buy SDR-35 or Schedule 40 PVC drain pipe in standard 10 ft or 20 ft lengths with gasketed bell joints. Cutting short pieces on site wastes material and fittings. Cross-checking the calculator’s output against a supplier quote helps catch differences in pricing assumptions — ask for exact product specifications (grade, finish, batch number) and confirm delivery timescales against your programme.
Adjusting the defaults
Every input in this calculator is editable. Enter your own dimensions, supplier prices, and wastage allowance — the output recalculates instantly. If the defaults feel off for your region or project type, your own numbers always override them.
Using this french drain calculator alongside other BuildMetricLab tools
This calculator works best as part of a planning workflow. Pair the quantity with our project contingency, labor-hours, and material-cost calculators to build a complete estimate before you pick up the phone to a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the math.
Sources & methodology
This tool estimates the three main material quantities for a French drain installation: crushed stone (gravel), perforated pipe, and filter fabric. Trench excavation volume is length x width x depth (all converted to feet). Gravel volume is the excavation volume minus the cylindrical displacement of the pipe (pi/4 x pipe diameter^2 x length), converted to tons using a bulk density of approximately 100 lb/cu ft for crushed stone (2,000 lb per ton). Pipe quantity equals the trench length. Filter fabric area is the trench cross-section perimeter (width + 2 x depth) multiplied by the length, with a 10% allowance for overlap at seams.
Frequently asked questions
Are french drain calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, compaction, and cut-offs all affect the true figure.
How much extra gravel or fabric should I order?
This calculator does not add a separate waste allowance for gravel, but geotextile area already includes a 10% overlap allowance for seams. Many contractors still round gravel and pipe orders up by 5-10% to cover compaction and off-cuts.
Can this tool replace a licensed contractor or design professional?
No. This tool is a planning estimator only. For work that affects structure, building code compliance, gas, electrical, plumbing, or drainage to a public sewer, consult a licensed contractor or design professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
Spotted something off?
Calculations or display — let us know.
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