A French drain is only as effective as the aggregate surrounding its perforated pipe. If you backfill a drainage trench with unwashed bank-run gravel or crusher run containing fine silt, groundwater will wash those fines straight into your perforated pipe slots and clog the system within a single season.
The 3 Golden Rules for French Drain Aggregate
- Always Specify "Washed / Clean" Stone: Your stone must be washed at the quarry to remove rock dust. Ask specifically for Washed #57 Crushed Stone (3/4"–1") or 1.5" Washed Round Drain Rock.
- Line the Trench with Non-Woven Geotextile Fabric: Before pouring any gravel, line the excavated soil trench with 4 oz to 6 oz non-woven drainage fabric (overlapping at the top like a burrito) so native soil cannot contaminate your clean stone.
- Maintain a 1% Minimum Slope: Drop the trench floor at least 1 inch for every 8 to 10 linear feet (1% grade) toward your daylight outlet or sump basin.
French Drain Gravel Chart (Per 50 Linear Feet of Trench)
| Trench Width × Depth | Base Cubic Feet | Cubic Yards (+10% Adj) | Tons of #57 Stone | 50-lb Bags |
|---|---|---|---|---|
| 12" Wide × 12" Deep (50' Long) | 50.0 ft³ | 2.04 yd³ | 2.75 tons | 110 bags |
| 12" Wide × 18" Deep (50' Long) | 75.0 ft³ | 3.06 yd³ | 4.13 tons | 165 bags |
| 18" Wide × 18" Deep (50' Long) | 112.5 ft³ | 4.58 yd³ | 6.19 tons | 248 bags |
| 18" Wide × 24" Deep (50' Long) | 150.0 ft³ | 6.11 yd³ | 8.25 tons | 330 bags |
Use our specialized French Drain Gravel Calculator to estimate custom trench lengths and depths in seconds.