French Drain Calculator
Get the gravel, pipe and fabric needed for a french drain trench.
What you'll need to order
- #57 washed drainage stone
- 4 in perforated pipe
- non-woven filter fabric
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
gravel yd³ = (length × (width ÷ 12) × (depth ÷ 12) − length × 0.0873) ÷ 27
The 0.0873 subtracts the cross-sectional area of a 4 inch pipe. Use washed stone with no fines — the whole point is that water moves freely through the voids, which crusher run prevents.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ASTM D2729 (Poly(Vinyl Chloride) Sewer Pipe and Fittings) 2023 — perforated drain pipe dimensions, from which the pipe displacement is subtracted
The edition matters as much as the number. Jurisdictions adopt code revisions on their own schedule, so verify which one applies where you are building.
Worked example
With trench length 50 ft, trench width 12 in, trench depth 18 in, this calculator returns 2.62 cu yd.
Gravel for common french drain runs
Open any row to load it into the calculator above — the link carries the numbers with it, so it can go straight to a supplier or a crew.
| Job | Size | Drainage gravel needed |
|---|---|---|
| Downspout run to daylight | 30 ft × 12 in × 18 in | 1.57 cu yd |
| Along a foundation wall | 50 ft × 12 in × 24 in | 3.54 cu yd |
| Wet lawn intercept | 80 ft × 12 in × 18 in | 4.19 cu yd |
| Deep curtain drain | 60 ft × 18 in × 36 in | 9.81 cu yd |
| Short patio edge drain | 20 ft × 10 in × 12 in | 0.55 cu yd |
Washed stone with no fines. Crusher run defeats the purpose — the voids are what move the water.
Frequently asked questions
How much gravel do I need for a french drain?
A 50 ft trench 12 inches wide and 18 inches deep takes about 2.6 cubic yards of washed stone. Multiply length by width and depth in feet, subtract the pipe volume, then divide by 27.
What gravel should I use in a french drain?
Clean washed #57 stone, roughly 3/4 inch, with no fines. Crusher run and road base contain dust that clogs the voids and turns the drain solid within a season.
How deep should a french drain be?
Eighteen to 24 inches for surface water. To intercept groundwater or protect a foundation, go below the water source — often 3 ft or more. The trench must also fall at least 1% along its run.
Do I need filter fabric around a french drain?
Yes. Without it, soil migrates into the stone and the drain silts up within a few years. Wrap the whole trench — line it before adding stone, then fold the fabric over the top before backfilling.
Should the pipe holes face up or down?
Down. Water rises into the pipe from the gravel bed below, and downward-facing perforations let it enter at the lowest point. Holes facing up means water must fill the trench before it can drain.