How this french drain calculator works
A french drain is a gravel-filled trench around a perforated pipe. The stone is trench volume times the drainage-stone density; the pipe and fabric follow the run:
pipe = length · fabric = length × (width + 2 × depth + overlap)
At the defaults — a 50 ft drain, 12 in wide and 18 in deep — that is 4.25 tons of clean #57 stone, 50 ft of pipe and about 250 ft² of fabric, roughly $291 in materials; the full chain is in the worked example. Clean stone and fabric are what keep a drain draining.
The parts of a french drain
A drain that lasts is four things working together — take one away and it silts up:
- Perforated pipe: the fast lane at the bottom of the trench, holes down, sloped to daylight.
- Clean stone: washed #57 around the pipe so water moves through the voids — priced on the gravel calculator.
- Filter fabric: non-woven geotextile wrapping the stone so soil can’t wash in and clog it.
- Slope: about 1% fall to an outlet — a dry well, daylight, or a storm connection.
- Outlet: where the water actually goes — the drain is only as good as its exit.
Trench size, stone, and slope
The trench sets the stone; the details set whether it works:
- Yard drain: 12 in wide, 18 in deep is the common size for surface and downspout water.
- Footing drain: deeper, alongside the footing, to relieve water against a foundation.
- Clean stone only: #57 washed — no crusher run, no fines, or the drain packs and fails.
- Slope 1%: an inch of fall every 8–10 ft, running to a lower outlet.
- Fabric always: line the whole trench and overlap the top before backfilling.
Where a french drain goes decides its size as much as the water does. A downspout drain can be short and shallow; a curtain drain intercepting a slope runs long and follows the contour; a footing drain rings the foundation at the base of the wall. Each is the same trench math — length, width and depth — but the outlet is what they share: find the fall to a lower point first, because a drain with nowhere to go is just a buried trench that stays wet.
Stone by drain length, and the right grade
Table 1 runs a 12 × 18 in trench out to common lengths; Table 2 is why the stone has to be clean.
| Length | Stone |
|---|---|
| 25 ft | 2.25 t |
| 50 ft | 4.25 t |
| 75 ft | 6.25 t |
| 100 ft | 8.25 t |
| Stone | Drainage? |
|---|---|
| #57 clean | Yes — the standard ✓ |
| #4 clean | Heavy flow ✓ |
| Pea gravel | OK, migrates |
| Crusher run | No — fines clog ✗ |
How to measure for a french drain
- Measure the drain run from the wet area to the outlet, following the low line.
- Set the trench width and depth — 12 × 18 in for a yard drain, deeper for a footing.
- Leave 10% waste for over-dig and the stone that settles into the subgrade.
- Read the stone, pipe and fabric, and confirm you have the fall to an outlet.
- Add the outlet and any dry well from the exclusions below.
Worked example: a 50 ft french drain
The default, worked line by line — a 50 ft drain, 12 in wide, 18 in deep, 10% waste, $45 a ton, $2 a foot of pipe:
stone = 2.78 × 1.10 × 1.35 = 4.13 → 4.25 tons
pipe = 50 ft · fabric = 50 × (12 + 36 + 12) ÷ 12 = 250 ft²
cost = $191 stone + $100 pipe = $291
Double the depth to 24 in — a footing drain — and the stone jumps to 5.5 tons for the same length, because volume grows with depth. The fabric grows too, since it has to wrap a taller trench. Stone is usually the biggest line, which is the reason to size the trench to the water, not to habit.
What this french drain estimate doesn’t include
The stone, pipe and fabric are the trench; a working drain needs a few more things:
- The outlet. A dry well, pop-up emitter, or connection to daylight — where the water goes.
- Fittings. Couplings, tees and a catch basin at a downspout tie-in.
- Excavation. Digging the trench — hand or machine, plus spoil haul-off.
- Backfill and sod. Restoring the surface over the drain.
- Delivery. Hauling the stone, real money on a bigger drain.
French drain FAQ
How much gravel do I need for a french drain?
Trench volume × the stone density: a 50 ft drain in a 12 in wide, 18 in deep trench is about 2.78 cubic yards, or 4.25 tons of clean #57 stone with 10% waste. Use washed stone with no fines — the fines are what clog a drain — and the calculator sizes it from your trench dimensions.
How deep and wide should a french drain be?
A yard drain is typically 12 in wide and 18 in deep; a footing or foundation drain goes deeper, down beside the footing. Wider and deeper moves more water but buys more stone fast. Whatever the size, slope the pipe about 1% — an inch of fall every 8 to 10 ft — to daylight or a dry well.
What kind of gravel goes in a french drain?
Clean, washed #57 stone — ¾ to 1 in with the fines screened out — so water moves freely through the voids. Never use crusher run or anything with dust in it: the fines migrate and pack, and a drain full of fines stops draining. The size chart covers the grades.
Do you need fabric for a french drain?
Yes — wrap the trench and the stone in a non-woven filter fabric so soil cannot wash into the voids and silt up the drain. The default 50 ft trench takes about 250 ft² of fabric to line the sides, bottom and overlap the top. Skipping it is the most common reason a french drain fails.
How much does a french drain cost?
Materials for the default 50 ft drain are about $291 — $191 in stone and $100 in pipe, plus fabric. Installed, a french drain commonly runs $10 to $30 per linear foot for a simple yard drain, and more for deep footing drains or where a machine has to dig hard ground.
How much gravel for a 100 foot french drain?
A 100 ft drain in a 12 × 18 in trench takes about 8.25 tons of #57 stone with 10% waste, plus 100 ft of perforated pipe. That is a big pile — over half a tandem load — so it is worth confirming the trench size and getting the stone delivered rather than hauled.
Sources and standards
The volume is exact; the drainage practice is long-settled:
- Clean-stone drainage practice: washed #57 around perforated pipe, wrapped in non-woven fabric.
- Standard estimating density: 1.35 t/yd³ for clean #57 — the site-wide drainage-stone figure.
- Slope: about 1% (⅛ in per foot minimum) to a lower outlet.
- The outlet governs: a drain with nowhere to go doesn’t drain — plan the exit first.
Embed this french drain calculator on your site
Run the french drain calculator on your own site, free. Paste the snippet where you want it; the visible credit link is the only condition.