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 |
French drain or another drain?
"French drain" gets used for everything, but each problem has a best fix:
- French drain. A perforated pipe in gravel that collects groundwater rising from below — for soggy yards and seepage.
- Channel / trench drain. A surface grate that catches runoff across a driveway or patio.
- Curtain drain. A French drain placed uphill of an area to intercept water before it arrives.
- Footing drain. A French drain around the foundation footing to keep basements dry.
- Dry well. A buried pit that collects and disperses the water a drain carries away.
Where to send the water
A drain only works if it has somewhere to go. Run it to daylight on a lower part of the lot, into a dry well, or to an approved storm outlet — and keep it sloping the whole way. Never drain toward a neighbor or back at the foundation, and check local rules before tying into any public system.
How much does a French drain cost?
DIY, the materials — washed stone, perforated pipe, and fabric — are the main cost; gravel runs about $30–$70 per ton. Professionally installed, a French drain typically runs roughly $10–$30+ per linear foot depending on depth, access, and whether it ties into a dry well.
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.