French Drain Calculator — Gravel, Pipe & Fabric
Enter your trench size and pipe diameter to get how much gravel your french drain needs (the calculator subtracts the pipe), plus filter fabric and pipe length.
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How it works
gravel = trench volume − pipe volume = L×(w÷12)×(d÷12) − π×(pipe_dia÷24)²×L (cu ft)
Gravel for a french drain by trench size (4 in pipe)
| Trench (L × W × D) | Gravel (cu yd) | Gravel (tons) |
|---|---|---|
| 25 ft × 12 in × 18 in | 1.31 | 1.83 |
| 50 ft × 12 in × 18 in | 2.62 | 3.66 |
| 50 ft × 12 in × 24 in | 3.54 | 4.96 |
| 100 ft × 12 in × 18 in | 5.23 | 7.33 |
| 100 ft × 18 in × 24 in | 10.79 | 15.10 |
Gravel volume already subtracts a 4-inch perforated pipe at ~1.4 tons per cubic yard. Use clean, washed ¾-inch stone, and line the trench with filter fabric.
Frequently asked questions
How much gravel do I need for a french drain?
A common 50 ft trench, 12 inches wide and 18 inches deep, with a 4-inch pipe needs about 2.6 cubic yards (≈3.7 tons) of gravel. The calculator subtracts the pipe’s volume automatically.
What kind of gravel goes in a french drain?
Use clean, washed ¾-inch crushed stone (often sold as #57 stone). Avoid fine gravel, pea gravel, or stone with fines, which clog over time and defeat the drainage.
Do I need landscape fabric in a french drain?
Yes — line the trench with permeable filter fabric before adding gravel and pipe, then fold it over the top. It keeps soil and roots from silting up the stone. Allow about 1.2× the trench’s wetted surface for overlap.