Free French drain calculator. Enter trench length, width, depth, and pipe diameter to get the gravel volume (net of pipe displacement), gravel weight, and total materials cost for gravel, pipe, and landscape fabric.
Use this before digging a French drain around a foundation, along a downspout run, or across a soggy yard — it converts your trench dimensions straight into a gravel order (by weight and volume), plus pipe and fabric footage, so you don't under-order mid-project.
Unlike a flat gravel-volume calculator, this tool nets out the pipe's displaced volume and converts gravel to both cubic yards (for ordering bulk delivery) and tons (how most gravel yards actually price and weigh it).
Trench volume comes from length × width × depth (converted to feet), giving the total cubic feet of the excavated channel.
The perforated pipe's own volume (π × radius² × length) is subtracted from the trench volume, since gravel only fills the space around the pipe.
Gravel weight is the remaining volume times your gravel's density, converted to tons, then priced at your local per-ton rate alongside linear-foot costs for pipe and fabric.
Most residential French drains are dug 18–24 inches deep, which puts the pipe below the frost line in most climates and below typical foundation footings. Shallower trenches work for simple surface-water diversion.
Fabric wraps the gravel and pipe to keep silt and soil from washing in and clogging the void spaces between the stones. Without it, a French drain can silt up and stop draining within a few years.
Aim for roughly a 1% grade — about 1 inch of drop for every 8 feet of trench — so water flows toward the outlet by gravity. This calculator estimates materials only; check your slope separately with a string level or laser level.
The perforated pipe itself takes up space inside the trench, so the gravel only needs to fill the space around it. Ignoring the pipe's volume would overestimate how much gravel you need to buy.
Entrée
50 ft trench, 12 in wide, 18 in deep, 4 in pipe, gravel $45/ton at 100 lb/cu ft density, fabric $0.50/ft, pipe $1.20/ft.
Sortie
≈2.62 cu yd (3.53 tons) of gravel; total materials cost ≈$243.94.
A 50 ft × 12 in × 18 in trench holds 75 cu ft; the 4-inch pipe displaces about 4.4 cu ft, leaving 70.6 cu ft of gravel (2.62 cu yd, 3.53 tons at 100 lb/cu ft) costing $158.94, plus $60 pipe and $25 fabric for $243.94 total.
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