This French drain calculator estimates the gravel, perforated pipe and fabric for a trench drain — net of the pipe’s own volume — from trench length, width and depth.
Slope is the whole game: a minimum 1% fall (⅛” per foot) toward the outlet — a level French drain is a French pond. Gravel spec: washed, angular ¾–1½” drain rock (not pea gravel, whose round faces pack and halve the void space water lives in). Line the trench with non-woven geotextile, overlap seams 12″, and leave enough fabric to fold over the top like a burrito before backfilling.
Trench Volume Minus Pipe — the Gravel Is the Drain
The pipe is almost a formality: a gravel trench alone drains (the original French drain had no pipe); pipe adds a fast channel inside the stone reservoir. So gravel math — gross trench volume less pipe displacement — is the purchase that matters, and our Gravel Calculator prices the driveway-grade quantities in the same units. Part of our Construction Calculators collection.
Worked Example (Verified)
- 50 ft × 12″ × 18″ (defaults): gross 75 ft³ (2.78 yd³) − pipe 4.36 ft³ → 2.62 yd³ gravel net; fabric ≈ 250 ft² with the wrap allowance
- The void-space dividend: drain rock at ~40% voids gives this trench ≈ 28 ft³ (210 gallons) of underground storage before the pipe even flows — the buffering that flattens storm surges
- Depth vs width economics: going 18″ → 24″ deep adds 33% more gravel and storage; widening does the same with easier digging — trenchers and soil decide
- Outlet math: the drain is only as good as its daylight point or dry well; volume calculations for a receiving dry well reuse this same net-volume math
Installation Truths
- Call before you dig (811 in the US) — yard utilities live exactly at French-drain depths.
- Perforations down (the common convention for drainage pipe in stone: water rises into the holes as the stone saturates; solid-top keeps the channel clear).
- Keep roof runoff separate or sized-for: a downspout can overwhelm a yard drain designed for seepage.
- Clay soil doubles the case for fabric: fines migration is the standard failure mode, turning stone into concrete over a decade.
Related Tools
- Gravel Calculator — bulk gravel by area and depth
- Retaining Wall Calculator — drains behind walls
- Construction Calculators — the full collection
Frequently Asked Questions
How much gravel do I need for a French drain?
Trench volume minus pipe volume: a 50-ft trench at 12″ × 18″ with 4″ pipe needs ≈ 2.6 yd³ of washed drain rock. This calculator nets out the pipe and adds the fabric wrap estimate. Buy ~10% extra for trench irregularities.
How deep should a French drain be?
Typically 18–24″ for yard drainage (below the saturated surface layer, above utility conflicts), with the outlet end’s depth set by your slope: 1% fall over 50 ft consumes 6″ of elevation — design from the outlet backwards. Foundation perimeter drains sit at footing depth, a different spec.
What size pipe for a French drain?
4″ perforated is the residential standard; 3″ suffices for short, light-duty runs; 6″ for long runs or combined flows. Corrugated is easier around curves; smooth-wall PVC/SDR flows better and can be snaked clean — the eternal trade-off.
Do I really need fabric?
In sandy soil, arguably optional; in clay or silt, it’s the difference between a 30-year drain and a 7-year one. Non-woven geotextile (not landscape weed fabric, which blinds) wrapped burrito-style around the stone is the professional default.
French drain vs trench drain — same thing?
No: a French drain is subsurface and gravel-filled (intercepts groundwater and percolation). A trench/channel drain is a surface grate system (intercepts sheet flow on driveways and patios). This calculator is the subsurface kind; name confusion derails many material orders.
Is this French drain calculator free?
Yes — free, instant, no sign-up.