French Drain Calculator
Calculate gravel, landscape fabric, and pipe for your French drain project. Get cubic yards, tons, and a material cost estimate.
Quick Answer
A 50 ft French drain trench at 12" wide × 18" deep needs about 2.78 yd³ of drainage gravel (~3.9 tons), 52 linear ft of landscape fabric, and 53 ft of 4" perforated pipe — totaling roughly $155–$330 in materials.
Trench Width
Trench Depth
Enter your trench length to calculate materials.
💡 Add 10% to gravel for settling. Always call 811 before digging.
Slope Check
Verify your trench has enough grade for gravity flow. Minimum 1% (1" drop per 8 ft run).
Minimum drop needed
Enter a run distance to see minimum drop requirements.
How to Calculate French Drain Materials
Formula
Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards
Convert trench width and depth from inches to feet before multiplying. Divide the total cubic feet by 27 to get cubic yards. Multiply cubic yards by 1.4 to get tons of drainage gravel (crushed stone #57 or #8 is standard). Order 10% extra to account for settling and uneven trench walls.
Example
A 50 ft trench that is 12" wide (1 ft) × 18" deep (1.5 ft): 50 × 1 × 1.5 ÷ 27 = 2.78 yd³. At 1.4 tons per yd³, that's about 3.9 tons of crushed stone. The landscape fabric wraps the full trench interior — for this trench, you'd need fabric at least 5 ft wide (1 ft bottom + 1.5 ft each side + 1 ft overlap) and 52 linear feet.
Tips & Best Practices
- •Always call 811 before digging — buried utilities are the #1 French drain mistake.
- •Use crushed stone #57 or washed drainage gravel — do NOT use pea gravel or round stone, which can compact and block flow.
- •Maintain 1% slope minimum (1 inch of drop per 8 feet of run). More is better — 1–2% is ideal.
- •Wrap the gravel in landscape fabric to prevent soil migration and clogging. Leave the top open.
- •For high-volume drainage, use a 6" perforated pipe instead of 4".
- •The outlet (daylight end) must discharge somewhere — a ditch, dry well, or drainage swale.
- •Order 10% extra gravel — trench walls are never perfectly flat.
Gravel Needed by Trench Size
| Trench Size | Per 10 ft | Per 50 ft | Per 100 ft |
|---|---|---|---|
| 6" wide × 12" deep | 0.19 yd³ | 0.93 yd³ | 1.85 yd³ |
| 12" wide × 18" deep | 0.56 yd³ | 2.78 yd³ | 5.56 yd³ |
| 12" wide × 24" deep | 0.74 yd³ | 3.70 yd³ | 7.41 yd³ |
| 18" wide × 24" deep | 1.11 yd³ | 5.56 yd³ | 11.11 yd³ |
Frequently Asked Questions
How much gravel do I need for a French drain?+
Use the formula: Length (ft) × Width (ft) × Depth (ft) ÷ 27 = Cubic Yards. A 50 ft trench at 12" wide × 18" deep needs 2.78 yd³ (~3.9 tons). A 100 ft trench at the same size needs 5.56 yd³ (~7.8 tons). Always add 10% extra for settling.
What size trench do I need for a French drain?+
A standard residential French drain trench is 12" wide × 18–24" deep. For serious drainage issues (high water table, heavy clay soil), go 18" wide × 24–36" deep. The minimum practical depth is 12" — anything shallower rarely provides adequate drainage.
What type of gravel is best for a French drain?+
Use crushed stone #57 or washed drainage gravel — angular crushed stone with no fines (dust or small particles). Never use pea gravel, river rock, or pea stone as the primary fill — round stones don't interlock and can shift, reducing drainage. Always wrap gravel in filter fabric to prevent soil infiltration.
Do I need pipe in a French drain?+
For most residential drainage problems, yes. A 4" perforated pipe in the trench dramatically improves flow capacity. Without pipe (a "trench drain" or "rubble drain"), gravel alone can handle light drainage but will clog faster over time. Use 6" pipe for high-volume situations like downspout drainage or wet yards near foundations.
How much does a French drain cost?+
DIY materials for a 50 ft French drain typically run $150–$350 (gravel + fabric + pipe). Professional installation costs $20–$50 per linear foot, or $1,000–$2,500 for a 50 ft drain — mostly labor. Permits may be required if the drain discharges to a roadway or waterway.
How deep should a French drain be near a foundation?+
Foundation French drains should be at least 24" deep — deep enough to intercept water before it reaches the footing. For serious basement water issues, the drain should run at or slightly below the footing level (typically 36–48" deep). This is usually a job for a professional waterproofing contractor.
Common French Drain Applications
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