French drain calculator

A French drain is a trench lined with filter fabric, holding a perforated pipe surrounded by clean stone, with the fabric folded over the top. The strip of fabric you need is the trench’s inside perimeter plus an overlap on top.

How a French drain works

A French drain gives water an easier path than the soil around it. The trench is lined with filter fabric, a perforated pipe sits near the bottom, clean stone fills in around and over the pipe, and the fabric folds over the top of the stone. Water seeps through the fabric, runs through the gaps between the stones, gets into the pipe and flows downhill to an outlet. Washington State University Extension describes exactly this build — fabric-lined trench, perforated pipe in gravel, fabric wrapped over the top and covered with soil — and recommends sloping the trench 0.5–1%.

stone in place = length × (trench width × stone depth − pipe area)
stone to order = stone in place × compaction factor × (1 + waste) × loose density
fabric strip width = 2 × stone depth + 2 × trench width + overlap
pipe = trench length

Worked through for the defaults: a 40 ft trench 12 in wide and 18 in deep with a 4 in pipe holds about 2.1 cubic yards of stone in place, which comes to about 3.2 tons of #57 once you allow for it settling a little and 5% waste. The fabric strip has to line the bottom (12 in), both sides (18 in each), cover the top (12 in) and overlap by 12 in — 72 in, so a 6 ft wide roll.

The stone: clean, with no fines

Use washed stone with no dust in it — #57 (about ¾ in) is the usual choice for yard drains, and larger clean stone such as #3 carries more water. Crusher run or anything with stone dust is the wrong material here: the fines fill the gaps and the drain clogs. Penn State’s Center for Dirt and Gravel Road Studies makes the same point for road underdrains, which use clean, uniform stone wrapped in geotextile. Clean stone barely compacts, so the calculator only adds about 5% for settling.

The fabric: wrap the stone

The fabric is what keeps a French drain working for years. Without it, fine soil washes into the stone and slowly silts it up. Penn State’s guidance for underdrains prefers a non-woven geotextile in most soils. Lay one strip along the trench, pushed into the corners, fill with stone, then fold both edges over the top so they overlap. That is why the strip is so much wider than the trench: a 12 × 18 in trench needs a strip about 6 ft wide. Standard landscape fabric rolls are often 3–4 ft wide, so look for drainage fabric in wider rolls, or join two strips with a generous overlap.

Pipe, slope and outlet

• Pipe: 4 in perforated pipe suits most yard drains; 6 in carries more water. Penn State’s underdrain bulletin lists 4 or 6 in pipe.
• Slope: WSU Extension recommends 0.5–1% for the trench; Penn State says underdrains should have at least 1%. One percent is 1 ft of fall per 100 ft, about ⅛ in per foot.
• Outlet: the water has to come out somewhere lower — a daylight outlet on a slope, a dry well, or a rain garden, well away from your foundation and your neighbour’s property.
• Solid pipe: from the end of the trench to the outlet, switch to non-perforated pipe so the water doesn’t leak back out along the way. Enter its length under More options.
• Buying it: rigid pipe usually comes in 10 ft lengths and flexible corrugated pipe in coils (50 and 100 ft are common) — pick yours and the calculator rounds up and lists the end cap, couplers and outlet fitting. Add an elbow or tee wherever the run turns.

Before you dig

In the US, call 811 a few days before you dig so buried utility lines are marked — a French drain trench is exactly the kind of long, deep cut that finds them. The soil that comes out takes up more room loose than it did in the ground; the excavation calculator tells you how much there will be to haul away.

For how clean stone differs from base stone, see gravel types for driveways and paths; for the weight of a yard of stone, the landscape material weight chart.