French Drain Gravel: How Much Stone You Need

A French drain works because water can move through clean stone and into the pipe. If you skimp on gravel, the system clogs or drains badly. Size the trench and stone before you dig.

French Drain Basics

A French drain has three parts:

1. Trench - Usually 6-12 inches wide, 12-24 inches deep
2. Perforated pipe - Sits at the bottom
3. Drainage gravel - Surrounds the pipe

The gravel does the heavy lifting. It prevents soil from clogging the pipe.

Standard Dimensions

Drain Type	Width	Depth	Pipe Size
Light duty	6 in	12 in	3 in
Standard	8 in	18 in	4 in
Heavy duty	12 in	24 in	4-6 in

The Formula

Gravel (cubic yards) = Length × Width × Depth ÷ 27

All measurements in feet

Example: 50-Foot French Drain

Dimensions:

• Length: 50 feet
• Width: 8 inches (0.67 feet)
• Depth: 18 inches (1.5 feet)

Calculation:

50 × 0.67 × 1.5 = 50.25 cubic feet
50.25 ÷ 27 = 1.86 cubic yards

Add 10% waste: 1.86 × 1.10 = 2.05 cubic yards

Quick Reference Table

Length	6” × 12”	8” × 18”	12” × 24”
25 ft	0.46 cu yd	0.93 cu yd	1.85 cu yd
50 ft	0.93 cu yd	1.86 cu yd	3.70 cu yd
75 ft	1.39 cu yd	2.78 cu yd	5.56 cu yd
100 ft	1.85 cu yd	3.70 cu yd	7.41 cu yd

Includes 10% waste factor

Best Gravel Types

Not all gravel works for drainage. Use these:

#57 Stone (Recommended)

• Size: 3/4 to 1 inch
• Round, washed stone
• Best water flow
• Won’t compact

#4 Stone

• Size: 1.5 to 2.5 inches
• Good for heavy flow
• Leaves larger gaps
• Works around larger pipes

River Rock

• Size: 1 to 3 inches
• Decorative option
• More expensive
• Good for visible drains

What NOT to Use

• Crusher run (compacts, blocks water)
• Pea gravel (too small, shifts)
• Limestone (deteriorates over time)
• Dirty unwashed stone (has fines)

Complete Material List

For 50-foot standard French drain:

Material	Quantity	Est. Cost
#57 gravel	2 tons	$80-100
4” perforated pipe	50 feet	$40-60
Landscape fabric	75 sq ft	$20-30
Pipe fittings	As needed	$10-20
Total		$150-210

Gravel Weight

Gravel is often sold by the ton. Here’s the conversion:

Gravel Type	Weight per Cubic Yard
#57 stone	2,700 lbs (1.35 tons)
#4 stone	2,800 lbs (1.4 tons)
River rock	2,600 lbs (1.3 tons)

Example: 2 cubic yards of #57 = 2 × 1.35 = 2.7 tons

Landscape Fabric

Always wrap gravel in fabric. It keeps soil out.

Fabric Calculation

Fabric sq ft = Trench Length × (Width + Depth × 2) + 20%

Example: 50-foot drain, 8” wide, 18” deep

50 × (0.67 + 1.5 × 2) = 50 × 3.67 = 183 sq ft
183 × 1.20 = 220 sq ft

Fabric Types

• Non-woven - Best for drainage, prevents soil penetration
• Woven - For heavy loads, less drainage
• Use: Non-woven 4-6 oz weight

Installation Steps

1. Dig Trench

• Slope 1% minimum (1 inch per 8 feet)
• Smooth the bottom
• Remove roots and rocks

2. Lay Fabric

• Line entire trench
• Leave excess on sides
• Overlap seams 6 inches

3. Add Base Gravel

• 2-3 inches of stone
• Level the base
• Check slope with level

4. Install Pipe

• Holes face DOWN
• Connect sections securely
• Extend to outlet point

5. Cover with Gravel

• Fill around and over pipe
• Leave 4-6 inches below grade
• Maintain consistent depth

6. Wrap and Backfill

• Fold fabric over gravel
• Top with soil or decorative stone
• Grade surface away from drain

Slope Requirements

French drains must slope toward the outlet.

Slope	Inches per 10 Feet
1% (minimum)	1.2 inches
2% (recommended)	2.4 inches
3% (steep)	3.6 inches

Example: 50-foot drain at 2% slope

• Drop: 50 × 0.02 = 1 foot (12 inches)

Common Mistakes

1. Wrong gravel - Crusher run clogs the system
2. No fabric - Soil enters and blocks pipe
3. Not enough slope - Water sits and backs up
4. Holes up - Pipe clogs with sediment
5. Too shallow - Surface water bypasses drain

When to Go Bigger

Use 12” wide, 24” deep for:

• High water table areas
• Heavy clay soil
• Large roof runoff
• Areas with standing water
• Basement waterproofing

Cost Comparison

DIY vs Pro	DIY Cost	Pro Cost
50-ft drain	$200-300	$1,000-2,500
100-ft drain	$400-600	$2,000-5,000

DIY can save a significant amount versus hiring a crew, but it takes a weekend of hard work.

Use Our Free Calculator

Our Gravel Calculator handles the math:

• Enter trench dimensions
• See cubic yards and tons
• Get cost estimates
• Choose gravel type

For fill dirt and topsoil needs, try our Dirt Calculator.

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Key Takeaway: A standard 50-foot French drain needs about 2 cubic yards (2.7 tons) of #57 washed gravel. Use non-woven landscape fabric. Slope at least 1% toward the outlet. Don’t use crusher run or pea gravel.

References

• ICC International Residential Code (IRC) (foundation drainage concepts): https://codes.iccsafe.org/
• OSHA (excavation and trenching safety): https://www.osha.gov/trenching-excavation