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.
Worked example
With trench length 50 ft, trench width 12 in, trench depth 18 in, this calculator returns 2.62 cu yd.
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.