How to calculate French drain gravel
Example: a 60 ft trench, 1 ft wide and 18 inches deep is 60 × 1 × 1.5 = 90 cubic feet = 3.33 cubic yards — about 4.5 tons of #57 stone.
Typical French drain size
| Part | Typical |
|---|---|
| Trench width | 8–12 in |
| Trench depth | 18–24 in |
| Pipe | 4 in perforated, holes facing down |
| Slope | At least 1% (about 1 in per 8 ft of run) |
| Stone | Clean washed stone such as #57, ¾–1½ in |
Pipe and fabric
You need about as much pipe as trench length, plus fittings. For filter fabric, each foot of trench needs a strip as wide as the trench bottom plus both sides plus about a foot of overlap on top — for a 1 ft × 1.5 ft trench, roughly 5 ft wide.
Call 811 before digging to have underground utilities marked.
Frequently asked questions
How much gravel do I need per foot of French drain?
For a 12 in wide, 18 in deep trench: 1.5 cubic feet per foot — about 150 lb of stone per foot.
What is the best gravel for a French drain?
Clean, washed angular stone like #57 (¾–1 in). Avoid stone with fines, which clogs the drain.
How deep should a French drain be?
Usually 18–24 inches deep and 8–12 inches wide, sloping at least 1% toward the outlet.
Do I need fabric in a French drain?
Yes — wrapping the gravel in filter fabric keeps soil from clogging the stone and pipe.
Related calculators
These figures are estimates for planning only. Material weights and coverage vary by supplier, moisture and compaction. Always confirm quantities and prices with your supplier before ordering. See our terms & disclaimer.