French Drain Calculator
Gravel, perforated pipe, and filter fabric for a french drain trench.
8 to 12 in for a yard drain; wider alongside a footing.
Deep enough to slope downhill the whole run; footing drains go deeper.
4 in is standard. The gravel figure subtracts what the pipe displaces.
Use clean, washed stone. Fines clog the drain.
Delivered gravel price. Leave at 0 to skip the cost estimate.
You need
2.09 cubic yards
Pipe volume already subtracted. Order a little extra; trenches never measure out exactly.
- Approximate weight
- 2.93 tons
- 50 lb bags
- 114 bags
- Perforated pipe
- 40 linear ft
- Filter fabric
- 200 sq ft
- Spoil to remove
- 2.22 cubic yards
Estimates only. Verify quantities before ordering.
How this is calculated
Gravel = (width x depth x length), minus the pipe's cross-section times length, converted to cubic yards by dividing by 27. Tons = cubic yards x density. Filter fabric width = trench width + 2 x depth + 12 in of overlap, times the length. Spoil removed = the full trench volume. The pipe cross-section uses its nominal diameter.
Enter the trench length, width, and depth and pick your pipe size, and this calculator returns the drainage gravel in cubic yards and tons, the perforated pipe, the filter fabric to wrap the trench, and the soil you will dig out. The gravel figure subtracts the volume the pipe takes up, so you are not paying for stone that the pipe displaces.
A french drain is a gravel-filled trench that gives water an easy path to follow downhill, usually around a perforated pipe at the bottom. Size the trench to the job: a shallow 8 inch wide trench handles surface and yard water, while a footing or foundation drain runs deeper and wider alongside the footing.
Materials for common french drains
Rows are 12 in wide and 18 in deep with 4 in perforated pipe and #57 stone, except the footing drain. Order a little extra gravel beyond these figures.
| Trench | Gravel (cu yd) | Tons | Fabric (sq ft) |
|---|---|---|---|
| 20 ft yard drain | 1.05 cubic yards | 1.47 tons | 100 sq ft |
| 40 ft yard drain | 2.09 cubic yards | 2.93 tons | 200 sq ft |
| 60 ft yard drain | 3.14 cubic yards | 4.40 tons | 300 sq ft |
| 100 ft yard drain | 5.23 cubic yards | 7.33 tons | 500 sq ft |
| 60 ft footing drain (12 x 36 in) | 6.47 cubic yards | 9.06 tons | 480 sq ft |
Choosing the gravel
The gravel is the drain; the pipe only speeds it along. What you want is clean, washed, angular stone with large open voids, which is why 3/4 inch #57 crushed stone is the standard choice. Angular stone interlocks and stays where you place it on a slope, and because it has been washed, there are no fines to pack into the gaps and choke off flow. Round washed river rock also works and is sometimes chosen where the trench is left exposed as a decorative dry creek.
Avoid anything with stone dust or fines. Crusher run, dense-grade, and #411 are made to compact into a hard surface, the exact opposite of what a drain needs: the fines fill the voids, water stops moving, and the drain silts up within a season or two. If a supplier offers washed versus unwashed at a small price difference, pay for washed every time on a drain.
Filter fabric and pipe
Non-woven filter fabric wrapping the gravel is cheap insurance against the most common failure: fine soil washing into the stone voids and clogging the drain. Line the open trench, fill it, and fold the fabric over the top like a burrito before capping with soil or sod. In clean sandy ground you can skip it; in silt or clay it is the difference between a drain that lasts decades and one that dies in a few years. A pipe sock adds a second layer of protection in the dirtiest soils.
For the pipe, rigid perforated PVC resists crushing and cleans out with a snake, while corrugated flexible pipe is cheaper and bends around obstacles but is harder to clear if it clogs. Either way, lay it with the holes facing down and keep a continuous downhill slope to a daylight outlet, a dry well, or a storm connection. The whole system is only as good as that outlet: a drain that runs to a low spot with nowhere to go just moves the puddle.
Common questions
What kind of gravel goes in a french drain?
Clean, washed, angular crushed stone, usually 3/4 inch #57. It has large voids that carry water fast and interlock so they stay put. Never use material with fines or stone dust (crusher run, #411, pea gravel in fine blends): the fines pack together and silt up the drain, which is exactly what you are trying to avoid.
How much gravel do I need for a french drain?
Multiply the trench width by depth by length (all in feet) and subtract the pipe, then divide by 27 for cubic yards. A 40 ft trench 12 in wide and 18 in deep with 4 in pipe takes about 2.1 cubic yards, roughly 2.8 tons of #57 stone. Order a little extra, since hand-dug trenches never come out to exact dimensions.
Do I need filter fabric around a french drain?
In most soils, yes. Wrapping the gravel in a non-woven filter fabric keeps surrounding soil from migrating into the voids and clogging the drain, which is the main way french drains fail over time. Lay the fabric in the trench first, fill with gravel and pipe, then fold the excess over the top before capping. Skip it only in clean, sandy soil that will not migrate.
How much slope does a french drain need?
At least 1 percent, about 1 inch of fall for every 10 feet of run, and more is better. The whole trench bottom must drop continuously toward the outlet with no low spots, or water pools and the drain stops working. Check the fall with a string line and level before you add any gravel.
Which way do the holes on the pipe face?
Holes down. It seems backward, but water in the ground rises into the pipe from below, so slots facing down let the pipe fill and drain at the lowest point. Holes up would require the trench to flood before any water entered the pipe. Use a pipe sock in silty soil for extra protection.