French Drain Calculator
Calculate materials for French drain systems including gravel, perforated pipe, filter fabric, and excavation. Includes drainage capacity analysis and cost estimation.
Sample Projects - Click to Load
Backyard French Drain
50 ft standard yard drainage
Foundation Perimeter Drain
120 ft around house foundation
Hillside Curtain Drain
75 ft interceptor drain
Garden Drainage
30 ft shallow garden drain
Wet Basement Solution
80 ft deep perimeter drain
Driveway Edge Drain
60 ft along driveway
Commercial Drainage
200 ft heavy-duty system
Custom Project
Enter your own dimensions
Standard French Drain
Basic trench drain for yard drainage
📏 Trench Dimensions
🔧 Materials
⚙️ Additional Options
💰 Pricing
Planning estimates only. Verify with a licensed engineer or contractor before construction.
📋 Key Takeaways
- • Trench Volume | L × W × D (convert to cu yd ÷ 27)
- • Gravel | Trench Vol − Pipe Vol; Tons = cu yd × 1.4
- • Slope | Min 1% (1" per 8 ft); Drop = (Slope% ÷ 100) × L × 12
- • Pipe | Perforated 4" or 6"; length = trench length
Did You Know?
French drains redirect groundwater via perforated pipe in gravel-filled trench.
Source: Design
1% slope = 1" drop per 8 ft. Minimum for drainage.
Source: Slope
3/4" crushed stone drains better than round pea gravel.
Source: Gravel
Geotextile fabric prevents soil from clogging gravel.
Source: Fabric
Foundation drains: 18"×24" typical. Yard: 12"×18".
Source: Sizing
4" perforated pipe handles ~10 GPM per 100 ft.
Source: Capacity
What is a French Drain Calculator?
A French drain calculator determines materials for drainage: gravel volume, perforated pipe length, filter fabric, slope, and cost. Redirects surface and groundwater away from foundations and yards.
🔬 Key Concepts
Trench
L × W × D. 12–24" wide, 18–36" deep.
Gravel
Trench vol − pipe vol. ~1.4 tons/cu yd.
Slope
1% min. Drop = slope% × length × 12.
Fabric
Line trench; overlap 6–12".
How to Design a French Drain System
- 1
Plan Route
Water source to outlet; min 1% slope
- 2
Trench
12–24" wide, 18–36" deep. Yard: 12×18"; foundation: 18×24"
- 3
Materials
Gravel = trench − pipe; fabric; 4" or 6" perforated pipe
- 4
Install
Fabric → gravel base → pipe → gravel fill → wrap fabric
When to Install a French Drain
💧 Standard French Drain
Basic trench drain for yard drainage
Recommended: 12" × 18"
🏠 Foundation Drain
Protects foundation from water damage
Recommended: 18" × 24"
🌊 Curtain Drain
Intercepts surface and subsurface water
Recommended: 12" × 24"
🛡️ Interceptor Drain
Captures water before it reaches structures
Recommended: 18" × 30"
🌱 Shallow Surface Drain
Light-duty surface water management
Recommended: 8" × 12"
⬇️ Deep Drainage System
Heavy-duty underground drainage
Recommended: 24" × 36"
French Drain Formulas
Dimension Reference
Yard Drain
Foundation
Slope
Important Considerations
⚠️ Avoid
- • Flat or negative slope
- • Pea gravel (round, poor drainage)
- • Skipping fabric in clay soil
✓ Best Practices
- • 3/4" crushed stone
- • Perforations down; slope to outlet
- • Call 811 before digging
⚠️ Disclaimer: Estimates for planning. Verify slope, outlet, and local codes. Heavy clay or high water table may need engineer.
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