French Drain Calculator

A french drain needs a drop of at least 1 percent, which is about 1/8 inch for every foot of pipe, or 1 foot of drop over a 100-foot run. More slope moves water faster. Less than 1 percent leaves water standing in the pipe, and silt settles out and clogs it.

Southwest Florida lots are flat, so slope is rarely the hard part. The outlet is. The drain needs somewhere lower to end: a swale, a pop-up emitter at a lower grade, a dry well, or another discharge point you are allowed to use. On a long run, 1 percent means the trench gets deeper toward the outlet. A trench that starts 12 inches deep ends 24 inches deep after 100 feet.

In low lots the summer water table can sit close to the surface, and a deep trench fills with water that has nowhere to go. Set the grade with a string line and a level or laser, not by eye.

French drain cost is priced per linear foot, and the final number depends on trench depth, digging access, stone volume, pipe, and where the water outlets. We do not quote a flat per-foot price without seeing the yard, because two lots on one street can differ a lot.

Sandy soil digs easily, but trench walls cave in, so the work goes slower than it looks. What moves the price most is what is in the way: irrigation lines, tree roots, pavers, sidewalks, driveways, and tight side yards where a machine cannot fit. Hauling off the spoil and repairing sod add to it.

Call Dennis at Epic Horizons Landscaping, (941) 946-8403, and we will look at the yard and give you a price for that run.

Use washed gravel between 3/4 inch and 1-1/2 inch, such as washed #57 stone, which runs roughly 1/2 to 1 inch. Washed matters as much as size, because dust and fines in unwashed stone settle into the fabric and slowly clog the drain.

The stone also has to be bigger than the holes or slots in the pipe, so it cannot work its way inside. In sandy Florida soil, the filter fabric matters more than the stone size. Sand moves through any gap and fills the voids between the stones, and that is the usual way these drains fail.

Buy stone from a yard that sells it washed and ask for it by that name. Bagged or loose stone that looks clean can still carry a lot of dust.

A typical residential french drain is a trench 6 to 12 inches wide and 12 to 24 inches deep, with a 4-inch perforated pipe laid near the bottom and covered by gravel. Go wider and deeper for heavy flow or long runs. Length is set by where the water sits, not by a rule.

Bigger is not always better. A deeper trench costs more to dig, needs more stone, and is more likely to hit standing groundwater in a low yard. Most home drains stay at the shallow end and use a 4-inch pipe.

If downspouts feed the drain, or the drain collects water from a large roof or slope, the flow is higher. In that case use the wider trench and a pipe with a solid section that carries the water to the outlet.

Stone needed in cubic yards equals trench width in feet times depth in feet times length in feet, divided by 27. A 100-foot trench 1 foot wide and 18 inches deep is 150 cubic feet, about 5.6 cubic yards. Subtract the pipe, then add 10 percent for settling and waste.

A 4-inch pipe takes up about 0.3 cubic yards per 100 feet, so subtract that. Stone is often sold by the ton. Crushed stone commonly weighs around 1.3 to 1.5 tons per cubic yard, but the supplier's number is the one to use.

Standard french drain dimensions are a trench 6 to 12 inches wide, 12 to 24 inches deep, a 4-inch perforated pipe on a bed of 2 to 3 inches of gravel, and gravel filling to within 2 to 4 inches of grade. Filter fabric wraps the whole trench, and the bottom slopes at least 1 percent.

Allow extra fabric width. The fabric has to line the bottom, climb both walls, and fold over the top of the stone, so 100 feet of this trench uses a roll about 5 feet wide.

Keep the top layer open. Sod over the drain works if the stone below stays clean. Mulch or topsoil mixed down into the stone is what clogs it.

For a french drain, use washed rock about 3/4 to 1-1/2 inch across. Anything under 1/2 inch packs tight and drains slowly, and rock over 2 inches leaves big voids that let sand wash in. Avoid limerock, crushed base, and any rock that comes with fines or dust.

Limerock is common in Southwest Florida yards, and it is cheap, which is why people try it. The dust in it washes into the fabric and the drain slows within a few seasons. If you want a decorative cap on top, use 1 to 2 inch rock so leaves and mulch do not settle between the stones.

A french drain spec in short: 4-inch perforated pipe, washed 3/4 to 1-1/2 inch stone, non-woven filter fabric around the trench, at least 1 percent slope, and an outlet that daylights or ties into a legal discharge point. Check local rules before discharging anywhere.

Rigid PVC holds its grade better in soft sand than corrugated pipe, which can sag. Plastic pipe also does not rust in salt air. I do not know of one rule that covers every city, county, and HOA here. Some require approval for drainage work or limit where water can discharge, so ask before you dig. Call 811 before digging, too.

A trench drain is a surface channel with a grate, sized by flow, not by trench dimensions. Multiply paved area in square feet by rainfall in inches per hour by 0.0104 to get gallons per minute. A 1,000-square-foot patio in a 4-inch-per-hour downpour sheds about 42 gallons per minute.

This assumes all the rain runs off, which is close to true for concrete and pavers. Compare the result to the flow rating the channel manufacturer publishes, and size the outlet pipe to carry it. Summer storms here can drop several inches an hour in short bursts, so size for the heavy rain, not the average.

A trench drain collects water on the surface and moves it away. A french drain collects water under the ground. Near salt air or a pool, plastic or composite grates hold up better than bare steel.

To calculate french drain gravel, multiply trench width by depth by length in feet, divide by 27 for cubic yards, subtract about 0.3 cubic yards per 100 feet of 4-inch pipe, and add 10 percent. For 100 feet of 12 by 18 inch trench, order about 5.8 cubic yards.

Stone is sold by weight, and the weight per yard varies with the stone and how wet it is. The 1.4 tons per cubic yard above is an assumption, so use your supplier's figure. Ordering a little over is cheaper than a second delivery, and leftover stone is useful for the outlet or a splash pad.