Plan a French drain from the trench dimensions. The calculator subtracts the perforated pipe from the trench volume to give the drainage gravel in cubic yards, cubic meters, and tons, then sizes the filter fabric that wraps the gravel, counts pipe sections or coils, and tells you how much the trench must fall from end to end at the slope you choose.
Buying stone for other parts of the yard? The gravel calculator works by area and depth. For water off the roof, size gutters and downspouts with the gutter size calculator, and check solid pipe capacity with the pipe volume calculator. If the water is coming down a slope, a retaining wall calculator estimate includes the drain pipe and gravel behind the blocks.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench length by its width and depth to get the trench volume, then subtract the space the pipe takes up (π × radius² × length). A 50 ft trench 12 in wide and 18 in deep holds 75 cubic feet; a 4 in pipe displaces about 4.4, leaving about 70.6 cubic feet, or 2.6 cubic yards. At roughly 2,700 lb per cubic yard for washed 3/4 in stone, that is about 3.5 tons. Add 5–10% for settling and uneven trench walls.
How wide and deep should a French drain trench be?
Most residential French drains are 6 to 12 inches wide and 18 to 24 inches deep, with the pipe resting on 2 to 3 inches of gravel and covered by more gravel above. Go deeper when intercepting groundwater along a foundation or a slope, and wider for heavy clay soils that shed a lot of water. The pipe must fit comfortably with gravel on every side, which is why the calculator rejects a pipe as wide as the trench. Call 811 before digging to have buried utilities marked.
How much slope does a French drain need?
The common minimum is 1%, about 1/8 inch of fall per foot, or 1 inch every 8 feet. Over a 50 ft run that is about 6 inches from the inlet end to the outlet. Steeper slopes of 2% move water faster and are more forgiving of small dips. The trench must fall continuously toward a daylight outlet, dry well, storm connection where permitted, or catch basin — never toward a foundation — and a low spot in the middle will hold water.
Do I need landscape fabric in a French drain?
Filter fabric (non-woven geotextile) keeps fine soil from washing into the gravel and clogging the drain. Line the bottom and both sides of the trench, add the gravel and pipe, and fold the fabric over the top before backfilling, overlapping it by at least the trench width. The calculator sizes the fabric width as trench width + 2 × depth + that fold-over. A perforated pipe with a fabric sock is an alternative, but wrapping the whole gravel bed lasts longer in silty soils.
Which way should the holes in a French drain pipe face?
Holes down is the usual advice for perforated rigid pipe: water rises into the gravel from below and enters the pipe as the level climbs, and fewer fines settle inside. Corrugated pipe is often slotted all the way round, so orientation doesn't matter. Either way, the pipe sits on a bed of gravel, not directly on soil. Plan cleanouts at the upper end and at bends so you can flush the line later. A catch basin at the low end collects debris before it reaches the outlet.