A French drain is a gravel-filled trench, usually with a perforated pipe, wrapped in filter fabric. Enter the trench size and pipe diameter; the calculator gives the drain gravel, pipe and fabric.
French drain formulas
Trench volume = length × width × depth Gravel = trench volume − pipe volume (π × radius² × length) Fabric to wrap the trench = length × (2 × depth + 2 × width)
Worked example: 50 ft trench, 12 in wide, 18 in deep, 4 in pipe
- Trench: 50 × 1 × 1.5 = 75 cu ft.
- Pipe volume: 50 × π × (2 ÷ 12)² = 4.36 cu ft.
- Gravel: 70.6 cu ft = 2.62 cu yd.
- Fabric: 50 × (3 + 2) = 250 sq ft, before overlap.
Sizing the trench
Trench width and depth vary with the job: a shallow drain for surface water can be narrow, while one intercepting groundwater may be deeper. The 12 × 18 in default is only an example, not a standard. The trench needs a steady fall toward the outlet; the slope calculator helps check it. Call 811 before digging to have buried utilities marked.
Converting to tons
Gravel suppliers often sell by the ton. Washed drain rock weighs roughly 1.3–1.5 tons per cubic yard depending on the stone; the gravel calculator explains the weight it uses and converts yards to tons.
Questions
How much gravel for a 100 ft French drain?
About 5.23 cu yd for a 12 × 18 in trench with 4 in pipe.
Do I need fabric?
Fabric keeps soil from clogging the gravel and is commonly used; add 10–20% for overlaps.
What size gravel?
Clean, washed stone with no fines, often 3/4 to 1½ in, so water drains freely.
Sources
- NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Cubic foot and cubic yard definitions.