Drainage
How Much Gravel Do I Need for a French Drain?
French drain gravel comes down to the trench volume minus the pipe. Measure the trench length, width and depth, pick a clean washed stone, and subtract the space the pipe takes up. Here's the method and a worked example.
The basic calculation
A French drain trench is a simple box, so the gravel volume is length × width × depth, then subtract the pipe:
Gravel volume = (Length × Width × Depth) − Pipe volume
A 4-inch perforated pipe takes up only about 0.087 ft³ per foot of trench, so on a narrow trench you can often ignore it; on a tight trench it's worth subtracting.
Worked example
A trench 30 ft long, 12 in wide, 18 in deep:
- Volume = 30 ft × 1 ft × 1.5 ft = 45 ft³
- Less a 4 in pipe (≈0.087 ft³/ft × 30 = 2.6 ft³) ≈ 42 ft³
- That's 42 ÷ 27 ≈ 1.6 cubic yards, or about 1.7 tonnes of clear stone.
What gravel to use
Use clean, washed, angular stone with no fines — commonly 3/4 in (19–20 mm) clear stone. The lack of stone dust keeps the voids open so water flows freely. Avoid crusher run here: its fines pack tight and choke the drain.
- Wrap the trench (and ideally the pipe) in landscape/filter fabric to keep soil out of the stone.
- Lay the perforated pipe with holes facing down on a thin bed of stone.
- Surround and cover the pipe with stone, then fold the fabric over the top.
Estimate your drain
Enter your trench dimensions in the French Drain Gravel Calculator for the volume and tonnage, and use the Crushed Gravel Calculator if you're ordering a specific clear-stone size. Not sure how deep to dig? See the gravel depth guide.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply the trench length × width × depth for the volume, then subtract the pipe. For example, a 30 ft trench that's 12 in wide and 18 in deep needs about 1.6 cubic yards (roughly 1.7 tonnes) of clear stone.
What kind of gravel goes in a French drain?
Clean, washed, angular stone with no fines — typically 3/4 inch (19–20 mm) clear stone. The open voids let water flow. Avoid crusher run, whose stone dust clogs the drain.
Do I need landscape fabric in a French drain?
Yes, it's strongly recommended. Wrapping the stone (and pipe) in filter fabric keeps surrounding soil and silt from washing into the gravel and clogging the drain over time.
How deep should a French drain be?
Most French drains are about 12–24 inches deep and 6–12 inches wide, sized to the pipe and how much water you're moving. The trench should slope at least 1% (about 1 inch per 8 feet) toward the outlet.
Disclaimer: This guide gives general estimates for planning. Actual gravel quantities, weights and prices vary with material type, particle size, moisture, compaction, supplier and your province. Costs are material-only ballparks in Canadian dollars and exclude labour, delivery and taxes unless stated. Always confirm quantities and prices with your local supplier.