French Drain Calculator

How to Use This French Drain Calculator

Start with the application preset. It fills the trench width and depth with numbers that match how that drain is normally built, and you can type over them at any point. The preset is a starting point drawn from common practice, not a code requirement. Your soil, your frost depth and your local inspector all get a vote.

Trench length is the run from the high end to the outlet. Measure it along the path the water actually takes, following the low side of the yard. A drain that runs straight because it looks tidy on a sketch but cuts across a swale will sit half full of silt within a season.

Width and depth define the cross section. Width is the trench as dug, depth is measured from the surface down to the bottom of the stone. The calculator subtracts the top cover you specify, which is the soil or sod you put back over the stone, and works with the stone layer only.

The pipe dropdown handles the volume question. A 4 inch pipe takes up space that stone would otherwise fill, so the tool removes it from the order. That sounds like a rounding error on a short run and stops being one on a 200 foot drain. Choosing the no-pipe option covers a gravel-only trench, which is what a simple dry well or a shallow surface swale uses.

Slope sets the total fall. Pick the grade, and the tool multiplies it by the run, so a 50 foot trench at 1 percent falls about 6 inches. That number is the height difference your outlet has to sit below your inlet. If the ground does not offer that fall, the drain needs a different route rather than a different calculation.

Last come the three prices. Stone is sold by the ton or the cubic yard, pipe by the foot, and geotextile fabric by the square foot or by the roll. Set any price to zero to drop that line out of the budget if you already own the material.

How Deep and Wide Should a French Drain Be?

The rule that matters is simple. The bottom of the stone has to sit below the water you are trying to move, and the outlet has to sit below the whole run. Everything else is site detail.

Width is driven by the pipe, not by drainage capacity. A 4 inch pipe bedded with 4 inches of stone below and 4 inches on each side needs a 12 inch trench. Go narrower and you cannot bed the pipe, so the stone bridges over it and the pipe carries far less water than its diameter suggests. Go much wider and you are buying stone you do not need, because the water enters through the sides and the bottom of the trench, not through the top. Twelve inches is the sweet spot for residential work, with 10 inches workable for small leaders and 18 inches sensible on a hillside curtain drain where the stone doubles as a collection bed.

Depth depends heavily on what the drain protects. A footing drain has to sit below the top of the footing so water never reaches the foundation wall under pressure, and most codes want the pipe invert at least 12 inches below the basement slab. In cold country the frost line takes over and can push the trench past 4 feet, because a drain that freezes solid in January drains nothing in March. Yard and surface drains that only handle rain runoff can sit shallower, often 18 inches, as long as there is enough cover to keep a mower blade and a tiller out of the stone.

One more constraint sneaks up on people in clay. A drain in heavy clay collects water slowly, because water has to pass through the clay before it ever reaches the stone. That does not mean the drain fails, it means the collection rate is low and the trench often runs deeper than the same drain in sand, since the water table sits higher above the clay layer. If your soil holds water in a hole overnight, count on a deeper trench and a longer outlet run.

Stone, Pipe and Fabric: What Goes in the Trench

Three materials do three different jobs, and swapping one for a cheaper substitute is how a drain that should last 30 years dies in five.

Washed crushed stone is the collection medium. The angular faces lock together under load so the trench does not settle, and the washed part matters more than the size. Stone sold with crusher fines still in it will compact into something close to concrete once it gets wet and loaded, and water stops moving through it. Aim for the 1 to 2 inch range. That is the #3 or #4 size in most supplier catalogs, and it holds large enough voids that silt takes decades to fill them. Pea gravel is the wrong call here: it is rounded, so it shifts underfoot and in a wheel rut, and its uniform 3/8 inch size packs tightly enough that fines seal the surface.

Perforated pipe gives the water a fast path to the outlet once the stone has collected it. Holes face down, so water enters from the bottom of the trench where the flow is cleanest and sediment settles below the openings rather than into them. Four inch diameter covers almost every residential run up to about 200 feet. Rigid perforated PVC costs more per foot and holds its shape and its grade for decades. Corrugated HDPE costs less, ships in coils and is far faster to lay, but it flexes, which makes a consistent grade harder to hold, and it tends to crack in the 20 to 30 year window. Both work. Pick based on whether you value your weekend or the next owner's. For a trench under a driveway or a heavily loaded area, use rigid.

Non-woven geotextile is the filter. It wraps the stone and lets water through while holding fine soil out. Wrap the full trench: line the sides and bottom, lay the pipe and stone, then fold the fabric over the top with a 6 inch lap before you backfill. Two shortcuts are common and both fail. A fabric sock around the pipe alone leaves the stone above and beside the pipe exposed to silt from the backfill. The cheap black weed barrier sold for garden beds is a different product, and it blocks water instead of filtering it.

Getting the Slope Right

Gravity is the only pump a french drain has. A pipe laid dead level holds water, and standing water drops its sediment, which is the start of the end. The accepted minimum grade is one eighth of an inch per foot, which works out to roughly 1 percent. A quarter inch per foot, about 2 percent, is a better target on a residential drain because it moves water faster and leaves a margin for the settling that happens over the first few years.

The check that matters is the total fall. Multiply the grade by the run: 50 feet at 1 percent falls 6 inches, and the same run at 2 percent falls a foot. The outlet has to sit at least that far below the inlet. This is where a lot of ambitious drainage plans die, because a yard that looks like it slopes toward the street on a cloudy day often turns out to have less than a foot of usable drop over 80 feet of run. Measure it before you dig rather than after.

Two practical notes on holding grade. First, a string line and a line level are enough for a residential drain. Set the string at the inlet elevation, level it, then drop the far end by your total fall and re-check. Second, dig the trench bottom flat, then bed the pipe in stone and check the grade again after the pipe is down but before you bury it. Corrugated pipe in particular will happily follow the trench floor into a dip you did not notice.

Do not overdo it either. Steep grades above about 4 percent move water fast enough to scour, and on a long steep run the water arrives at the outlet carrying silt that then settles where the grade flattens. On a genuinely steep site, a series of gentler runs, or a drop structure at the outlet, beats one pipe chasing the hillside.

Worked Example: A 50 Foot Footer Drain

Take a house with a wet basement wall along one side. The plan is a footer drain running 50 feet, 12 inches wide, 18 inches deep, with 2 inches of soil and sod put back over the stone.

The stone layer is 16 inches thick, since 18 minus 2 is 16. Multiply the run by the width by that depth in feet: 50 times 1.0 times 1.333 gives 66.7 cubic feet of trench. Now take out the pipe. A 4 inch pipe has a radius of 2 inches, or 0.1667 feet, so its cross section is 3.1416 times 0.1667 squared, which is 0.0873 square feet. Over 50 feet that is 4.4 cubic feet of space the stone never fills. Subtract it and the stone order is 62.3 cubic feet.

Divide by 27 and you get 2.31 cubic yards. Washed crushed stone runs about 1.4 tons per cubic yard, so the order is 3.2 tons. Most suppliers round to the nearest half ton, and a 10 percent cushion for spillage and a trench that wanders wider than planned puts the practical order at 3.5 tons.

Pipe length matches the run at 50 feet, plus whatever it takes to reach the outlet. Fall at the 1 percent grade is 50 times 0.125, or 6.25 inches, which means the outlet has to sit at least 6 inches below where the pipe starts. Fabric is sized by the trench wrap: the width plus twice the stone depth plus 12 inches for the top lap comes to 12 plus 32 plus 12, or 56 inches, which is 4.67 feet of fabric for every foot of trench. Across 50 feet that is 233 square feet, or about one 4 by 100 foot roll with enough left over for the outlet trench.

At $45 per ton for stone, $2.50 per foot for 4 inch perforated PVC and $0.15 per square foot for geotextile, the materials come to roughly $145 of stone, $125 of pipe and $35 of fabric, about $305 before you pick up a shovel. Hiring it out typically multiplies that by three, because the labor is all digging, and digging is the part nobody wants to do.

5 French Drain Mistakes

1. Using pea gravel because it is easy to shovel. Rounded stone never locks together, so the trench settles under a wheelbarrow and the voids close up. Washed crushed stone is harder on the shovel and worth the effort. The same argument rules out anything sold with crusher fines still mixed in.
2. Skipping the fabric, or using the wrong fabric. A trench with bare stone fills with silt in a few seasons and stops draining while still looking exactly like a drain from above. Non-woven geotextile wrapping the whole cross section is the fix, and it costs about $35 on a 50 foot run.
3. Laying the pipe flat. One season of settling turns a level pipe into a low spot, and a low spot collects sediment until it blocks. Hold at least one eighth of an inch per foot and re-check the grade after the pipe is bedded and before the stone goes in on top.
4. No plan for where the water goes. A drain that ends in the middle of the lawn is a drain that makes a mud hole at the end of the pipe. Confirm the outlet first, whether that is daylight on a downhill slope, a pop-up emitter, a dry well or a permitted storm connection, and remember that discharging onto a neighbor's lot is a legal problem as much as a drainage one.
5. Digging too shallow next to a foundation. A trench that stops above the footing collects water and delivers it right to the wall it was supposed to protect. The bottom of the stone has to sit below the top of the footing, and in cold climates below the frost line as well. Depth is the one number you cannot correct later without digging the whole thing twice.