French Drain Gravel Calculator
Cubic yards and tons of gravel to fill a French drain trench.
Input sheet
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A French drain is a gravel-filled trench, and filling it takes a surprising amount of stone once you account for the depth. This calculator gives you the cubic yards and tons of drainage gravel to order.
How it works
Trench length × width × depth in feet, divided by 27. Drainage gravel weighs ~1.4 tons per cubic yard.
Volume is trench length × width × depth in feet (width and depth converted from inches), divided by 27 for cubic yards. Gravel is then converted to tons at about 1.4 tons per cubic yard, the typical weight quarries use for drainage stone.
If you're running a large-diameter perforated pipe, subtract its volume — the pipe displaces gravel. For a small pipe the difference is negligible, so the calculator gives the full trench volume.
Cubic yards = (length × (width ÷ 12) × (depth ÷ 12)) ÷ 27; tons = cubic yards × 1.4.
Worked examples
A 40 ft trench, 12 inches wide and 18 inches deep. → About 3.1 tons (≈ 2.2 cubic yards).
40 × 1 × 1.5 = 60 cu ft ÷ 27 ≈ 2.22 yd × 1.4 ≈ 3.1 tons.
A 60 ft trench, 18 inches wide and 24 inches deep. → About 9.3 tons.
60 × 1.5 × 2 = 180 cu ft ÷ 27 ≈ 6.67 yd × 1.4 ≈ 9.3 tons — confirm with supplier for the exact stone.
Tips & gotchas
- Wrap the gravel in landscape filter fabric so soil and fines don't clog the drain over time — this is what keeps a French drain working for decades.
- Use angular drainage stone (like #57) rather than rounded pea gravel; angular stone locks together and holds void space for water to flow.
- Slope the trench at least 1% (about an inch per 8 feet) so water actually moves toward the outlet.
- Subtract the pipe's volume only if you're using a large 4-inch-plus perforated pipe; for small pipe it's a rounding error.
FAQ
What kind of gravel goes in a French drain?
Angular, washed drainage stone such as #57 — about 3/4-inch clean stone. Its irregular shape holds void space for water to move, unlike rounded pea gravel that packs tight.
Do I need filter fabric?
Yes — wrapping the gravel and pipe in non-woven filter fabric keeps soil fines from migrating in and clogging the drain. It's the single biggest factor in how long the drain lasts.
How much slope does a French drain need?
At least 1%, or about an inch of drop per 8 feet of run, so gravity carries water to the outlet. More slope is fine; flat or reverse-sloped trenches hold water instead of draining it.
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