Planning a French drain? Our free drainage gravel calculator tells you exactly how many cubic yards of gravel your trench needs — automatically subtracting the space taken up by your drain pipe.
👤 Who is this for: drainage contractors, landscapers & wet-yard DIYers.
How to use
- Enter your trench length (feet), width and depth (inches).
- Select your pipe size (4 inch, 6 inch, or no pipe).
- Hit Calculate Drainage Gravel.
Drainage gravel formula
Gravel = [(Length × Width/12 × Depth/12) − pipe area × Length] ÷ 27. Pipe cross-sections: 4-inch pipe = 0.087 sq ft, 6-inch pipe = 0.196 sq ft.
Example: a 50 ft long × 12 in wide × 18 in deep trench with 4-inch pipe → (50 × 1 × 1.5) − (50 × 0.087) = 70.6 cu ft = 2.62 cubic yards.
Common French drain setups
| Setup | Trench size |
|---|---|
| Standard French drain | 12 in wide × 18 in deep |
| Deep drain | 24 in wide × 24 in deep |
| Narrow drain | 8 in wide × 12 in deep |
📝 Real Project Example
50 ft French drain, 12 in wide × 12 in deep:
- Trench volume: 50 × 1 × 1 = 50 cu ft
- Cubic yards: 50 ÷ 27 = 1.85 → order 2 cubic yards of #57 washed stone
- Weight: 1.85 × 1.4 = ~2.6 tons
- Plus: 50 ft of perforated pipe and fabric to wrap the trench
- Cost check: 2 cubic yards at ~$45 = ~$90 + pipe & fabric
Use washed angular stone — never pea gravel or rounded rock in a drain. Round stone shifts and fines clog the voids that carry water.
Tips
- Slope the trench about 1% (1 inch per 8 feet) toward the outlet.
- Use 3/4-inch clean (washed) gravel — no fines, so water flows freely.
- Wrap the trench in landscape fabric to stop silt from clogging the gravel.
📚 Sources & Methodology
How we calculate. Step 1: trench volume in cubic feet = length × width × depth (all in feet). Step 2: cubic yards = cubic feet ÷ 27. Step 3: tons = cubic yards × 1.4 (#57 washed stone). We recommend 6–12 inches of clean angular gravel around the perforated pipe, wrapped in filter fabric.
Cross-checked. Our depth and stone-type guidance matches published drainage references; the #1 French-drain failure is silt clogging from unwrapped trenches.
Sources:
- Free Asphalt Calculators — Gravel Calculator (6–12 in drainage depth guidance)
- Ergeon — Gravel Calculator (aggregate density references)
Last updated: October 2026.
Frequently asked questions
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