What a French drain is
A French drain is a trench lined with filter fabric, filled with washed stone, with a perforated pipe near the bottom. Water in the soil finds the stone easily, drops into the pipe and runs downhill to an outlet — a lower part of the yard, a dry well, or a pop-up emitter at the curb where allowed.
How deep and wide?
For yard drainage, 12" wide and 18"–24" deep is the usual size. The start has to be deep enough for about 2" of stone under the pipe, the pipe itself, 2"–3" of stone over it, and whatever soil or sod goes on top. A 4" pipe suits most homes; use 6" for long runs or a lot of water.
Slope
The pipe must fall at least 1% — about 1/8" per foot, or 1" every 8 ft — all the way to the outlet. 2% drains faster and is worth it where the outlet allows. Stretch a string line between stakes, level it, and measure down from it at both ends to see how much the ground already falls. If it falls less than the pipe needs, the trench gets deeper toward the low end; the planner shows by how much.
Stone, pipe and fabric
Use clean, washed stone between 3/4" and 1 1/2" with no fines, so water moves through it freely. Line the trench with non-woven geotextile fabric, fill with stone around the pipe, then fold the fabric over the top so soil can't wash in. Lay the pipe with the holes facing down. Rigid perforated PVC or SDR 35 is easier to keep on grade than corrugated pipe and is easier to clean out.
Where the water goes
Every drain needs an outlet lower than its low end. Never send the water onto a neighbor's property, and check local rules before draining to a street or storm sewer. Call 811 a few days before you dig so buried lines get marked.
Fixing roof runoff too? Size the gutters and downspouts with the gutter calculator.