CalculatorMulchThe pipe takes up half a yard you do not have to buy.
French drain
for a 40 ft run:
2.1 cubic yards of gravel
= 56.5 cu ft · 2.62 US tons · about 23 sq yd of filter fabric
- Trench volume
- 60 cu ft
- Taken up by the pipe
- 3.5 cu ft
- Fall needed at 1%
- 4.8 in over the run
No pipe means the whole trench is gravel. That works for a short soakaway and clogs faster than a piped drain in silty ground.
What the yard needs this month
MonthlyOne job a month, with the depth and the volume it actually needs. No plant catalogues, no upsell โ just the material side of the garden.
Try also
How much gravel does a French drain need?
The trench volume minus whatever the pipe displaces. A 40-foot run at 12 inches wide and 18 deep is 60 cubic feet of trench. A 4-inch perforated pipe running the length of it takes up 3.5 cubic feet, so the gravel is 56.5 cubic feet โ 2.1 cubic yards, about 2.6 tons. On a longer run with a 6-inch pipe the displacement passes a full cubic yard, which is a real line on a delivery ticket.
Two other numbers matter as much as the volume. Filter fabric has to wrap the whole trench and overlap at the top, so it is not the trench area โ it is the perimeter times the length plus the overlap, which for this run is about 34 square yards. And the fall: a French drain needs roughly 1% slope, so 40 feet needs about 5 inches of drop from one end to the other. A level trench full of gravel is not a drain, it is a bathtub.
Use angular washed stone of about ยพ inch, not pea gravel and not anything with fines. The voids between angular stones are what carries the water; rounded gravel packs tighter and fines silt the whole thing up within a couple of seasons.
What the pipe saves you
A 40-foot trench, 12 in wide and 18 in deep โ 60 cubic feet in total:
| Pipe | Displaced | Gravel needed | Tons |
|---|---|---|---|
| None | 0 cu ft | 60.0 cu ft (2.2 yd) | 2.8 |
| 3 in | 2.0 cu ft | 58.0 cu ft (2.1 yd) | 2.7 |
| 4 in | 3.5 cu ft | 56.5 cu ft (2.1 yd) | 2.6 |
| 6 in | 7.9 cu ft | 52.1 cu ft (1.9 yd) | 2.4 |
Small on a short run, real on a long one: 120 feet with a 6-inch pipe displaces almost a full cubic yard.
The three numbers that decide whether it works
| Number | Rule of thumb | 40 ft run |
|---|---|---|
| Fall | 1% minimum | 5 in drop |
| Stone size | ยพ in washed, angular | โ |
| Fabric | perimeter + 12 in overlap | about 34 sq yd |
Nearby
For a driveway or path rather than a trench, the gravel calculator works by area and depth. If you are backfilling the trench with soil above the gravel, fill dirt covers the top layer, and cubic yards to tons tells you whether it fits in your truck.
Frequently asked questions
What size gravel goes in a French drain?
Washed angular stone around ยพ inch โ sold as clean stone, No. 57 stone or 20 mm clean depending on where you are. Washed is the important word: anything with fines or dust in it will silt up and defeat the drain within a few years.
Do I need the pipe at all?
For anything over about 15 feet, yes. Gravel alone moves water slowly and clogs from the inside; a perforated pipe gives it a clear path and roughly ten times the flow. Short soakaways behind a wall can be gravel only.
Which way up do the holes go?
Down, which surprises most people. Water enters from below through the gravel and rises into the pipe; holes-down lets the pipe fill from the water table and start moving it sooner. Holes-up means the water must fill the trench before the pipe does anything.
How much fall does it need?
About 1%, which is roughly an inch every eight feet, or five inches over a 40-foot run. More is fine. Less and the water sits; a dead-level trench holds water in the gravel and the drain does nothing except get expensive.
Does it need filter fabric?
Wrapped around the gravel, yes โ it keeps soil fines out of the voids, which is what makes the difference between a drain that lasts twenty years and one that lasts five. Do not wrap the pipe itself in fabric sock unless the ground is sandy; in silt the sock is what blocks first.
How deep should the trench be?
Deep enough to sit below whatever you are draining. Eighteen inches is typical for a garden run, deeper against a foundation, and the outlet end must be lower than the inlet โ which usually decides the depth for you.
How we calculate this
Worked example
With Trench length 40, Width 12, Depth 18, Perforated pipe 4 in, this page works out 2.1 cubic yards of gravel. = 56.5 cu ft ยท 2.62 US tons ยท about 23 sq yd of filter fabric
- Trench volume60 cu ft
- Taken up by the pipe3.5 cu ft
- Fall needed at 1%4.8 in over the run
Sources:
The drain fails at the outlet, not the trench
French drains fail for three reasons and none of them is the gravel. They fail because there was no fall, because the fines got in, or because the outlet went nowhere. The trench is the easy part; everything that determines whether it works is decided before the first stone goes in.
The outlet is the one worth checking twice. A drain that ends in a soakaway pit in the same clay that caused the problem has moved the water twenty feet and solved nothing. It has to reach somewhere the water can actually go โ a lower part of the garden, a soakaway in permeable ground, or a legal connection to a surface water drain.