French Drain Gravel Calculator
Estimate how much gravel your French drain needs. Enter the trench length, width and depth to get cubic yards, tons and cost — pre-set to clean #57 stone, the standard free-draining fill.
Picks a typical depth & material — tweak anything below.
For a 20 × 1 ft area at 12 in deep, order about 0.81 cubic yards (1.04 US tons) of #57 gravel.
Estimates only. Densities vary by moisture, compaction and supplier — confirm quantities before ordering.
How much gravel do I need for a French drain?
Multiply the trench length by its width and depth (all in feet) to get the volume in cubic feet, then divide by 27 for cubic yards. A typical French drain trench is about 12 inches deep and 6 to 12 inches wide, wrapped in filter fabric around a perforated pipe and backfilled with gravel. For a 20 ft trench that's 1 ft wide and 12 inches deep, that's roughly 0.75 cubic yards. Enter your own trench size above for the exact amount.
What gravel is best for a French drain?
Use clean, angular, open-graded stone with no fines so water flows freely — #57 gravel (about 3/4 inch) is the standard, and #67 or 3/4-inch clean stone also work. Avoid gravel with sand or fines, which clogs the drain over time. Always line the trench with filter fabric to keep soil out of the stone.
How deep and wide should a French drain be?
Most French drains are 8 to 12 inches deep and 6 to 12 inches wide, sloped at least 1% (about 1 inch of fall per 8 feet) so water moves. Set the perforated pipe on a couple inches of gravel, then surround and cover it with more stone up to a few inches below the surface.
French drain gravel by trench depth (per foot of length, 12 in wide)
| Trench depth | Gravel per 10 ft | Gravel per 50 ft |
|---|---|---|
| 8 inches | ~0.25 cu yd | ~1.2 cu yd |
| 10 inches | ~0.31 cu yd | ~1.5 cu yd |
| 12 inches | ~0.37 cu yd | ~1.9 cu yd |
Assumes a 12-inch-wide trench filled with gravel. Subtract a little for the pipe if you want to be precise.
Worked examples
- 20 ft × 1 ft = 20 sq ft
- 20 × 12 in (1 ft) = 20 cu ft
- 20 ÷ 27 = 0.74 cubic yards
- × ~1.28 ton/yd³ ≈ 0.95 tons
- 50 ft × 1 ft = 50 sq ft
- 50 × 10 in (0.83 ft) = 41.7 cu ft
- 41.7 ÷ 27 = 1.5 cubic yards
- × ~1.28 ton/yd³ ≈ 2 tons
Common mistakes to avoid
- Using gravel with fines. Stone with sand or dust clogs a drain within a few seasons. Use clean, open-graded #57 or similar with no fines.
- Skipping filter fabric. Without landscape/filter fabric lining the trench, soil migrates into the gravel and blocks flow. Wrap the trench (and often the pipe) in fabric.
- No slope. A flat trench holds water instead of moving it. Grade the trench at least 1% downhill to the outlet.
- Trench too shallow. A shallow drain can't carry much water. Most trenches are 8–12 inches deep so there's room for pipe plus gravel around it.
Frequently asked questions
How much gravel for a French drain?+
For a 20 ft trench 1 ft wide and 12 inches deep, about 0.75 cubic yards (roughly 1 ton) of #57 stone. Enter your trench length, width and depth above for the exact amount.
What size gravel for a French drain?+
Clean, angular #57 gravel (about 3/4 inch) is the standard; #67 or any clean 3/4-inch stone with no fines also works so water drains freely.
How deep should a French drain be?+
Usually 8 to 12 inches deep and 6 to 12 inches wide, sloped at least 1% to the outlet, with the perforated pipe bedded and covered in gravel.