French Drain Calculator

A French drain is a gravel-filled trench containing a perforated pipe that redirects surface and groundwater away from problem areas. Calculating materials before digging prevents costly over- or under-ordering.

The volume formulas: Trench Volume = Length × Width × Depth Gravel Volume = Trench Volume − Pipe Volume (pipe volume is usually negligible) Gravel Weight = Volume (cubic ft) × Gravel Density (lb/cu ft)

Standard pea gravel or crushed stone density: approximately 100 lbs per cubic foot (or 1.3–1.6 tons per cubic yard depending on stone type).

Conversion: 1 cubic yard = 27 cubic feet

Worked example: French drain: 60 ft long, 12 inches wide (1 ft), 18 inches deep (1.5 ft) Trench volume = 60 × 1 × 1.5 = 90 cubic feet = 3.33 cubic yards Gravel needed = 90 × 100 lbs = 9,000 lbs = 4.5 tons

Material cost estimate:

• Crushed stone (⅜ to ¾ inch): $30–$60/ton delivered → 4.5 tons ≈ $135–$270
• 4-inch perforated pipe: $0.50–$1.00/linear foot → 60 ft = $30–$60
• Landscape fabric (drain wrap): $0.15–$0.30/sq ft → ~90 sq ft = $14–$27
• Total materials: ~$180–$360
• Labor (if hired): $30–$60/linear foot → 60 ft = $1,800–$3,600

Design rules:

• Minimum slope: 1 inch of fall per 10 feet of run (1% grade) — gravity does the work
• Depth: At least 18–24 inches for yard drainage; 36–48 inches near foundation walls
• Pipe diameter: 4-inch for residential, 6-inch for high-volume areas
• Always terminate drain to daylight, a dry well, or a storm drain — never a sanitary sewer