French Drain Calculator

What this calculator does

Trench dimensions, pipe size and slope go in. Out comes the excavation volume in place and after swell, the drainage stone in cubic yards and tons net of the volume the pipe displaces, the linear feet of perforated pipe, the square feet of filter fabric needed to wrap the trench, the total fall across the run, and a warning if that fall is not enough to move water.

Four materials, four numbers, and one geometry check that decides whether the whole thing works.

The stone, and why the pipe is subtracted

A trench 60 feet long, 12 inches wide and 24 inches deep holds 120 cubic feet. A 4-inch pipe running its full length occupies about 5.2 cubic feet of that.

Small, but it is the kind of detail that separates an estimate from a guess, and on a large system with 6-inch pipe it stops being small. The calculator takes it out.

The stone converts to tons at about 1.4 tons per cubic yard, which is the standard dry-weight average for crushed stone. Yards sell by the ton, so that is the number you order against.

Get the right stone, which is not “gravel”

This is the single most common material mistake in residential drainage.

You want clean, open-graded, washed stone — typically 3/4 inch, with no fines. Its defining property is that there is lots of empty space between the pieces, and water moves through that space freely.

What you do not want is dense-graded aggregate, also sold as road base, crusher run, or just “gravel.” It contains a deliberate blend of sizes right down to stone dust, precisely so that it compacts into a dense, stable, nearly impermeable layer. It is the correct material under a paver patio and exactly wrong in a drain.

The two look broadly similar in a pile at the supply yard. Ask for washed stone by name, and look at it before it is dumped.

Slope: one percent, minimum

One percent is one foot of drop per hundred feet — about an inch every eight feet. That is the floor.

Below it, water moves slowly enough through the pipe that suspended silt has time to settle out of it. Settled silt reduces the pipe’s capacity, which slows the water further, which lets more silt settle. Drains that clog usually clogged because they were laid nearly flat.

The calculator reports the total fall across your run, which is worth looking at in absolute terms: a 100-foot drain at 1 percent drops a full 12 inches end to end. If the outlet end cannot physically be 12 inches lower than the inlet end, that run is too long for that slope, and the design needs rethinking rather than flattening.

Where does the water go?

This is the question that sinks more french drains than any material choice.

A drain does not destroy water. It moves it somewhere. That somewhere must be lower than the drain, must be able to accept the water, and must not be your neighbour’s yard — discharging concentrated runoff onto adjacent property creates real liability in most jurisdictions.

Legitimate outlets: daylighting on a downslope portion of the site, a storm sewer connection where local rules permit it, a dry well sized for the volume, or a rain garden built to absorb it.

Not an outlet: the pipe simply ending somewhere in the fill. That is a very expensive trench full of rock, and it will back up in exactly the storm you built it for.

Holes down

Perforated pipe has holes on one side. They face down.

The reasoning takes a moment. Water percolates down through the stone and collects at the bottom of the trench. With holes down, the pipe begins taking water as soon as the water level reaches the bottom of the pipe. With holes up, water has to rise past the entire diameter of the pipe before any of it gets in — so the trench floods to pipe height before the drain does anything.

Holes down. It is settled, and it is still argued about on every job site in the country.

Fabric: separation, not filtration

Filter fabric wraps the stone — bottom, both sides, and overlapped across the top before backfill. The calculator sizes it as trench width plus twice the depth plus a foot of overlap, which is the developed width of that wrap.

Its job is separation. Soil fines migrating into the voids between stones fill the very space that makes the drain work. It happens gradually and invisibly, and a drain that silts up from the outside gives no warning before it simply stops.

Use a non-woven geotextile. The cheap woven landscape fabric sold for weed control has very different permeability and is not the same product.

Note the one place fabric is contested: wrapping the pipe in a sock, in fine silty soils, can clog at the pipe rather than at the fabric. Wrapping the trench is the standard detail either way.

What this leaves out

• Where the water goes. Discussed above; the most important part.
• Catch basins, cleanouts and outlet protection. A cleanout at each end is cheap now and invaluable in ten years.
• Depth relative to what you are draining. A drain must sit below the water you are trying to remove — a footing drain goes at the footing.
• Utility locates. Call 811 first. The excavation calculator covers spoil volume and swell.
• Permits. Some jurisdictions regulate stormwater discharge.
• Sump pumps where gravity outfall is not available.

For the drainage behind a wall rather than across a yard, the retaining wall calculator sizes the stone chimney that does the same job vertically.