French Drain Gravel Calculator
Gravel needed for a French drain trench. Yards, tons and bags by trench length for a 1 ft wide, 12-inch deep trench.
A typical French drain trench is about 12 inches wide and 12 inches deep. Rows assume a 1 ft wide trench — widen it in the main calculator if yours is bigger.
Pea gravel at 12 in deep · density ≈ 1.45 tons/yd³ · 0.5 ft³ bags.
| Length × 1 ft | Cubic yards | Tons (est.) | 0.5 ft³ bags |
|---|---|---|---|
| 10 ft | 0.37 | 0.54 | 20 |
| 20 ft | 0.74 | 1.07 | 40 |
| 30 ft | 1.11 | 1.61 | 60 |
| 50 ft | 1.85 | 2.69 | 100 |
| 75 ft | 2.78 | 4.03 | 150 |
| 100 ft | 3.70 | 5.37 | 200 |
Tonnage is an estimate from loose, dry bulk density; damp or compacted material weighs more. Order ~5–10% extra for settling.
A French drain redirects groundwater away from a foundation, low spot, or waterlogged area by routing it through a perforated pipe surrounded by washed gravel. The gravel creates a highly permeable channel so water flows freely toward the pipe instead of pooling in saturated soil.
Use clean ¾-inch washed gravel (also called "drainage stone" or "pea gravel" at some suppliers) — the rounded stones leave large void spaces for water to move through. Avoid pea gravel mixed with fines, which can clog the voids. Wrapping the gravel in a filter fabric sock is recommended if you have silty soil to prevent long-term clogging.
Frequently asked questions
How much gravel do I need for a French drain?
A typical 1 ft wide × 1 ft deep trench needs about 0.037 cubic yards of gravel per linear foot, or roughly 1 cubic yard per 27 feet of trench. The table above shows exact figures by trench length.
What size gravel should I use for a French drain?
Clean ¾-inch washed gravel (also called drainage stone) is the standard. The uniform size creates large voids for fast water movement. Avoid mixed-size gravels with fine particles — they compact over time and block drainage.
How deep should a French drain be?
Typical residential French drains are 12–24 inches deep, with a fall of at least 1% (1 inch per 8 feet) toward the outlet. Deeper is needed if you are draining below a foundation footing. A 12-inch depth works for surface drainage and minor water table issues.