French Drain Calculator
Every other tool for this gives you a gravel number and stops. A trench also needs pipe, fabric and somewhere to fall to, so this french drain calculator turns your dimensions into the whole materials list, and it takes the space the pipe occupies back off the stone instead of charging you for it twice.
Your materials list
| Item | Spec | Quantity | Cost |
|---|---|---|---|
| Stone | #57 washed stone, 3/4" | 9.9 tons | $377 |
| Pipe | 4" corrugated, sock | 100 ft (1 × 100 ft) | $95 |
| Geotextile | 6 ft wide, use 6 ft × 300 ft | 600 sq ft | $150 |
| Material total | $622 |
What to order
9.9 tons #57 washed stone (7.1 cu yd), 100 ft of 4" corrugated, sock, 600 sq ft geotextile
$6 a foot in materials. Installed by a contractor the same run typically runs $20 to $30 a foot.
- The pipe takes up space
- A 4 in pipe displaces 8.7 cubic feet over 100 ft, so the stone figure above is the trench volume minus the pipe rather than the trench on its own. That is 4.4% of the trench, or about 0.45 tons you would otherwise have paid for twice.
- Setting the fall
- 12 in of drop across 100 ft, which is 1/8 in per ft. Run a string line from the high end and measure down to it as you dig. A drain that runs level holds water and one that runs backwards is worse than no drain at all.
- Why the fabric is that wide
- 72 in covers the trench floor, both walls and a 12 in overlap at the top, because the fabric wraps the stone rather than just lining the bottom. A 6 ft × 300 ft roll suits this trench.
- How this is worked out
- 100 ft × 12 in × 24 in ÷ 144 = 200 cu ft of trench− 8.7 cu ft of pipe = 191.3 cu ft ÷ 27 = 7.08 cu yd× 2,800 lb ÷ 2,000 = 9.92 tons
Calculate your french drain materials in one pass
The trench volume is length times width times depth. Keep the length in feet and the width and depth in inches, multiply the three together and divide by 144 for cubic feet, then by 27 for cubic yards. Multiply cubic yards by the stone’s weight per yard and divide by 2,000 for tons.
Then take the pipe out. A 4 in pipe running the full 100 ft of a 12 by 24 in trench occupies 8.7 cubic feet, which is 4.4% of the trench and about 0.45 tons of stone you would otherwise buy and have nowhere to put. On a long run with 6 in pipe it is over a ton.
Fabric is the piece most estimates miss entirely. It has to wrap the stone, not just line the bottom of the trench, so the width you need is the trench width plus both wall heights plus an overlap. For that same trench it works out at 72 in, or 6 ft, which is why a 3 ft roll is not enough for anything but the shallowest run.
How much gravel does a french drain need?
A 100 ft run at 12 by 24 in takes 7.1 cubic yards of stone, which is about 9.9 tons. That is roughly 0.1 tons for every foot of trench, and the figure scales directly with the cross-section.
| Trench | Use | Cubic yards | Tons of #57 | Fabric width |
|---|---|---|---|---|
| 12" × 18" | Footing / shallow | 5.2 | 7.3 | 5 ft |
| 12" × 24" | Standard yard | 7.1 | 9.9 | 6 ft |
| 18" × 30" | Wet yard | 13.6 | 19 | 7.5 ft |
| 18" × 36" | Basement perimeter | 16.3 | 22.9 | 8.5 ft |
Depth drives the number far harder than width does, because you are usually adding inches to a 12 in width rather than feet. Going from the standard 24 in yard trench to a 36 in basement perimeter almost triples the stone once the extra width comes with it.
Order about 10% over whatever the calculator says. Trench walls slump, the bottom is never perfectly flat, and stone settles as it is compacted around the pipe. Running short by half a ton on a Saturday afternoon costs a whole extra delivery.
What size gravel to use in a french drain
Three quarter inch washed stone, sold as #57 through most of the country. The size is not the important part, though. What decides whether a drain still works in ten years is whether the stone has been washed free of fines.
| Stone | Size | Lb per yd³ | Use it? | Why |
|---|---|---|---|---|
| #57 washed stone | 3/4" | 2,800 | Yes | The default. Washed clean of fines, widely stocked, packs without sealing |
| 1.5" clean stone | 1–1.5" | 2,750 | Yes | Bigger voids and the best flow. Harder to shovel and dearer per ton |
| Washed river rock | 1–2" | 2,700 | Yes | Rounded and decorative, so it works where the trench stays visible |
| Pea gravel | 3/8" | 2,700 | No | Voids are too small and it migrates. Fine as a top dressing, poor as the drain |
| Crusher run | 3/4" minus | 3,000 | No | Contains stone dust by design. It sets like concrete and the drain fails |
Best gravel for french drain runs
#57 washed stone is the default because it is stocked everywhere, cheap, and has enough void space to move real volumes of water. Step up to 1.5 in clean stone if the run is long or the water table is high, since bigger stone means bigger gaps between the pieces and faster flow. Below 3/4 in the voids start closing up.
French drain with river rock
Washed river rock at 1 to 2 in drains nearly as well as angular stone and looks considerably better where the trench is left open at the surface as a dry creek bed. The rounded shape packs slightly tighter, so allow a little more depth. Use it for the visible top few inches over #57 if you want the look without the cost.
Why crusher run and pea gravel fail
Crusher run contains stone dust deliberately, because it is made to compact into a solid surface. Put it in a trench and it does exactly that, setting into something close to concrete within a couple of seasons. Pea gravel has the opposite problem: the pieces are small and rounded, so they nest together and leave very little void space, and they migrate through fabric gaps over time.
Pipe and fabric quantities
Pipe is the easy one. It runs the full length of the trench, so the quantity is the trench length, and it comes in 100 ft rolls for corrugated or 10 ft sticks for rigid PVC. Buy one roll for every 100 ft and expect to use a coupler.
| Pipe | Sold in | Per ft | 100 ft run |
|---|---|---|---|
| 3" corrugated, sock | 100 ft | $0.75 | $75 |
| 4" corrugated, sock | 100 ft | $0.95 | $95 |
| 4" rigid PVC, perforated | 10 ft | $2.40 | $240 |
| 6" corrugated, sock | 100 ft | $1.90 | $190 |
Four inch corrugated with a sock is what most yard drains use, and it is the right default. Rigid PVC costs more than twice as much but can be rodded out if it ever blocks, which makes it worth the money under a driveway or anywhere digging it up again would be painful. Six inch is for genuinely large catchments.
Fabric quantity catches people out because the width is not the trench width. It lines both walls and the floor and folds back over the top, so a 12 by 24 in trench needs 72 in of width and a 600 sq ft roll for 100 ft. Use non-woven geotextile rather than the woven weed barrier sold in garden centers, which blocks almost immediately.
Trench depth, width and slope
Twelve inches wide by twenty-four deep handles most wet yards. Width below 8 in makes the trench hard to work in and holds too little stone to matter, and above 18 in you are usually better off going deeper instead.
Slope is the part that decides whether any of it works. One percent is the accepted minimum, which is 1/8 in per foot, or 12 in of drop across a 100 ft run. Set it with a string line from the high end and measure down as you dig, because eyeballing a fall that shallow across a long trench isn’t possible.
| Run length | 1% (minimum) | 2% | 3% |
|---|---|---|---|
| 25 ft | 3" | 6" | 9" |
| 50 ft | 6" | 12" | 18" |
| 100 ft | 12" | 24" | 36" |
| 150 ft | 18" | 36" | 54" |
| 200 ft | 24" | 48" | 72" |
Read the bottom row and the practical limit becomes obvious. A 200 ft run at 2% drops 48 in, so a trench that starts 24 in deep finishes over 72 in down. That is why long runs either stay at 1% or get broken into sections with a catch basin between them.
Materials list for a 100 ft drain
A standard 12 by 24 in yard drain, 100 ft long, with 4 in sock pipe and a fabric-wrapped trench, comes to $622 in materials, or $6 a foot.
| Item | Spec | Quantity | Cost |
|---|---|---|---|
| Drainage stone | #57 washed stone, 3/4" | 9.9 tons (7.1 yd³) | $377 |
| Perforated pipe | 4" corrugated, sock | 100 ft (1 roll) | $95 |
| Geotextile | Non-woven, 6 ft wide | 600 sq ft | $150 |
| Total | $622 |
Add a catch basin at the inlet if surface water is feeding it, a rodent grate at the outlet, and couplers for every pipe joint. None of them are expensive but all three get forgotten, and the outlet grate in particular is what stops the pipe becoming a nest within a year.
Stone is around 61% of that total, which is why the stone price field on the calculator matters more than any other input. A $10 swing per ton moves the whole job by $99.
Two deliveries is the other thing worth avoiding. 9.9 tons is a single tandem load with room to spare, so order the stone in one drop and stage it on a tarp near the trench rather than having it tipped twice. Haulage is charged per trip whatever the tonnage, and a second trip for half a ton of shortfall costs more than the stone in it.
What a french drain costs
Materials run about $6 a foot for a standard yard trench. Installed, a contractor will usually quote $20 to $30 a foot, so the same 100 ft run lands somewhere between $2,000 and $3,000 against $622 of materials.
The gap is labor and excavation, and excavation is where the quotes diverge. A trench a mini-excavator can reach is cheap. The same trench hand-dug because the gate is 30 in wide can double the price, and hitting tree roots, an old septic line or a buried cable pushes it further again. Get the utilities marked before anyone quotes, not after.
Doing it yourself saves most of that, and the machine is the decision. A day’s mini-excavator hire runs $250 to $400 and will trench 100 ft comfortably, against perhaps three days of hard digging by hand. Below about 40 ft the hire and transport cost stops being worth it; above 100 ft it always is.
French drain questions
How much does a french drain cost?
Materials for a 100 ft run at 12 by 24 in come to about $622, or $6 a foot. Installed by a contractor, expect $20 to $30 a foot for a straightforward yard run and $50 or more where the trench is deep, hand-dug, or has to get past tree roots and services.
How much does it cost to install a french drain?
Labor is most of it. A 100 ft yard drain typically runs $2,000 to $3,000 installed against roughly $622 of materials. Excavation is the variable: machine access makes it cheap, a narrow gate or a basement perimeter makes it expensive.
How much gravel do I need for a french drain?
Length times width times depth, converted to cubic yards, minus what the pipe takes up. A 100 ft run at 12 by 24 in holds 7.1 cubic yards, which is about 9.9 tons of #57 washed stone. The 4 in pipe accounts for 4.4% of the trench.
What size rock for french drain?
Three quarter inch washed stone is the standard, sold as #57 in most of the country. Anything from 3/4 to 1.5 in works as long as it is washed. The size matters less than the washing: what kills a drain is stone dust, not diameter.
Can I use river rock for a french drain?
Yes, if it is washed and 1 to 2 in. Rounded stone has slightly less void space than angular stone but drains perfectly well, and it looks better where the trench stays open at the surface. Do not use small decorative pebbles, which pack too tightly.
Do I need fabric around a french drain?
Yes in almost every soil. Geotextile keeps silt out of the stone, and silt is what eventually clogs a drain. Line the trench, fill it, then fold the fabric over the top before the topsoil goes back. Skipping it buys you a drain that works for a few years.
How deep should a french drain be?
24 in is standard for a wet yard. Shallower runs at 18 in handle surface water along a footing, and a basement perimeter goes to 36 in or below the footing. Depth matters less than fall: a deep drain with nowhere to discharge still holds water.
How much slope does a french drain need?
One percent minimum, which is 1/8 in per foot, or 12 in of drop across 100 ft. More is better where you have the depth for it. Below 1% the water moves too slowly to carry silt through and the stone starts to fill.
Does a french drain need a pipe?
Not always. A stone-only trench works for small surface water problems and is sometimes called a soakaway. Add pipe once the run is over about 20 ft or the water has to be taken somewhere specific, because stone alone moves water far more slowly than an open pipe does.
Which way do the holes face on french drain pipe?
Holes down. It looks wrong and it is right: water has to rise from the bottom of the trench into the pipe, so holes at the bottom let the pipe start draining the trench sooner and keep the water table below the stone rather than above it.