Multiply trench length × width × gravel depth in feet and divide by 27 for cubic yards, then multiply by 1.45 for tons of #57 stone. A 30-ft trench 12 inches wide filled 12 inches deep needs about 1.1 cubic yards, or 1.6 tons — a 4-inch pipe displaces only about 0.09 cubic feet per linear foot, so it barely dents the order.
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French Drain Calculator Guide
A French drain is a gravel-filled trench with a perforated pipe at the bottom, and the gravel is most of what you buy. This calculator works in trench terms — length, width, and the depth of stone — because that is how you will dig it. Enter the three dimensions and it returns cubic yards and tons of washed stone at #57’s density of 1.45 tons per cubic yard.
One number people agonize over needlessly: the pipe. A 4-inch perforated pipe displaces only about 0.09 cubic feet per linear foot, so even a 50-foot run subtracts less than a fifth of a cubic yard. Order to the full trench volume and treat the pipe’s displacement as part of your margin.
How Much Stone Does a French Drain Use Per Foot?
The table nets out the 4-inch pipe and converts at 1.45 tons per cubic yard for #57 stone.
| Trench (width × stone depth) | Stone per linear ft | Per 10 ft (yd³) | Per 10 ft (tons) |
|---|---|---|---|
| 6 in × 12 in | 0.41 ft³ | 0.15 | 0.22 |
| 12 in × 12 in | 0.91 ft³ | 0.34 | 0.49 |
| 12 in × 18 in | 1.41 ft³ | 0.52 | 0.76 |
| 18 in × 18 in | 2.16 ft³ | 0.80 | 1.16 |
The 12 × 12 configuration — half a ton per 10 feet — is the workhorse for yard drains. Go wider and deeper where the drain intercepts serious groundwater, such as at the base of a slope.
The Fabric Burrito, Step by Step
- Dig with fall: at least 1 inch of drop per 10 feet of trench, running to daylight, a dry well, or a storm connection.
- Line the trench with non-woven geotextile, leaving enough excess on each side to fold over the top later.
- Bed the pipe on 2–3 inches of washed stone, perforations facing down, so water enters from below.
- Fill with stone to within a few inches of grade, then fold the fabric over the top — the burrito that keeps soil out for decades.
- Cap with turf, topsoil, or decorative stone depending on whether the drain should disappear or read as a gravel band.
Use washed stone only — fines are the enemy of every drain. The #57 stone calculator covers the standard drain rock in more depth, and for surface drainage projects beyond trenches, the drainage gravel calculator handles beds, backfill, and under-slab layers.
Frequently Asked Questions
What stone should I use in a French drain?
Washed, open-graded stone only — #57 or #67 are the standards, and rounded river rock also works. Never use crusher run, stone dust, or anything with fines, because the fines clog the voids the drain depends on.
How much gravel does a French drain need per foot?
A 12-inch-wide trench filled 12 inches deep holds 1 cubic foot of stone per linear foot, minus about 0.09 cubic feet displaced by a 4-inch pipe. That nets out to roughly 0.05 tons of #57 stone per foot, or about half a ton per 10 feet.
Do I need landscape fabric around a French drain?
Yes — line the trench with non-woven geotextile, place the stone and pipe inside, then fold the fabric over the top before backfilling. This “burrito” wrap keeps soil from migrating into the stone and clogging the drain over time.
How wide and deep should a French drain trench be?
Most residential drains run 12 inches wide and 18–24 inches deep, with the pipe bedded on 2–3 inches of stone and at least 12 inches of stone in the trench overall. Maintain a fall of about 1 inch per 10 feet toward the outlet.