How much gravel for a French drain?
A French drain is a gravel-filled trench with a perforated pipe, so the gravel is just the trench volume minus the pipe:
Cubic yards = (length × width × depth − pipe volume) ÷ 27, then tons = cubic yards × 1.4
Measure width and depth in inches and divide by 12 to get feet. We subtract the volume the pipe occupies, divide by 27 for cubic yards, and multiply by about 1.4 to estimate tons — the typical weight of washed drainage stone. Bulk delivery is far cheaper than bags once you pass roughly a cubic yard.
Sizing a French drain
| Spec | Typical |
|---|---|
| Trench width | 8–12 in |
| Trench depth | 12–24 in |
| Slope | ≥1% (≈1 in per 8–10 ft) |
| Gravel | 3/4 in clean / washed |
| Pipe | 4 in perforated, holes down |
Tips that keep it draining
- Use clean stone, not pea gravel with fines. Fines pack down and choke the flow — washed angular stone stays open.
- Wrap it in fabric. Line the trench and fold the fabric over the top so silt can't migrate in and clog the gravel.
- Holes face down. Water enters the pipe from the bottom of the trench, then flows away inside it.
- Always slope to an outlet. Run the drain to daylight, a dry well, or a lower grade — a flat trench just holds water.