What size gravel is best for French drains?

3/4 inch to 1 inch clean crushed stone or river rock is ideal for French drains. This size provides excellent drainage while preventing pipe clogging.

How deep should a French drain trench be?

French drain trenches should be 18-24 inches deep and 12-18 inches wide. The pipe should sit on 2-3 inches of gravel with 8-12 inches of gravel covering it.

Do I need sand in a French drain?

Sand is optional but recommended as a 2-3 inch top layer before backfilling with soil. It prevents soil from washing into the gravel and maintains drainage efficiency.

How much gravel do I need per linear foot of French drain?

For a standard 18-inch deep, 12-inch wide trench, you need approximately 0.5-0.7 cubic feet of gravel per linear foot, or about 1.5-2 tons per 100 feet.

Can I use pea gravel for a French drain?

Pea gravel is not recommended for French drains. While attractive, its small size (1/4 to 1/2 inch) can clog pipe perforations and compact too tightly, reducing drainage efficiency. Stick with 3/4 to 1 inch drainage stone for reliable long-term performance. Pea gravel can be used as a decorative top layer if desired.

How long does a French drain last?

A properly installed French drain with quality materials lasts 30-40 years or more. Key factors affecting longevity include using filter fabric to prevent silt infiltration, proper gravel sizing, adequate slope for drainage, and regular maintenance. Systems without fabric or with improper materials may fail within 10-15 years due to clogging.

How to Calculate Gravel and Sand for French Drain | Complete Guide

Understanding French Drain Components

A properly installed French drain requires precise calculations of multiple materials. The system consists of a perforated pipe surrounded by drainage gravel, wrapped in filter fabric, and topped with sand before backfilling. Each component plays a crucial role in water management and system longevity.

The key to successful French drain installation lies in accurate material estimation. Too little gravel compromises drainage efficiency, while excess material increases costs unnecessarily. Our drainage stone calculator helps you determine exact quantities based on your trench dimensions.

Step-by-Step Calculation Method

1. Measure Your Trench Dimensions

Standard French drain trenches measure 18-24 inches deep and 12-18 inches wide. Measure the total length of your drainage run, including any curves or angles. For example, a 50-foot run with an 18-inch depth and 12-inch width provides your base measurements.

2. Calculate Gravel Volume

The gravel calculation formula is: Length × Width × Depth = Volume. Convert all measurements to feet first. For our example: 50 ft × 1 ft × 1.5 ft = 75 cubic feet. Since gravel compacts, add 10-15% extra: 75 × 1.15 = 86.25 cubic feet needed.

3. Determine Sand Requirements

Sand forms a 2-3 inch protective layer above the gravel. Calculate: 50 ft × 1 ft × 0.25 ft = 12.5 cubic feet of sand. This prevents soil infiltration while maintaining water flow to the gravel layer below.

4. Convert to Tons or Yards

Suppliers typically sell by cubic yards or tons. Divide cubic feet by 27 for yards: 86.25 ÷ 27 = 3.2 cubic yards of gravel. For tons, multiply cubic yards by 1.4 (average gravel weight): 3.2 × 1.4 = 4.5 tons. Use our stone tonnage calculator for precise conversions.

Material Selection Guidelines

Best Gravel Types for French Drains

3/4 inch clean crushed stone: Most popular choice, excellent drainage, angular shape prevents shifting
River rock (3/4 to 1 inch): Smooth, attractive, slightly more expensive but longer-lasting
Pea gravel: Not recommended - too small, can clog pipe perforations
Drainage rock: Specifically washed and sized for drainage applications

Sand Specifications

Use coarse construction sand or concrete sand for the top layer. Avoid fine play sand which can wash through the gravel. The sand should be clean and free of clay or organic matter that could impede drainage.

Installation Layer Breakdown

Proper French Drain Layering (Bottom to Top):

Base layer: 2-3 inches of gravel (foundation for pipe)
Perforated pipe: 4-inch diameter, holes facing down
Gravel surround: 8-12 inches covering pipe completely
Filter fabric: Landscape fabric wrapped over gravel
Sand layer: 2-3 inches of coarse sand
Topsoil: Native soil to grade level

This layering system ensures water flows efficiently to the pipe while preventing soil contamination. The filter fabric is critical - it allows water through while blocking fine particles that could clog the gravel over time.

Common Calculation Mistakes to Avoid

Forgetting Compaction Factor

Always add 10-15% extra gravel to account for settling and compaction during installation.

Using Wrong Gravel Size

Gravel smaller than 3/4 inch can clog pipe perforations, while larger than 1.5 inches creates gaps that allow soil infiltration.

Insufficient Pipe Coverage

Pipe must be completely surrounded by at least 3 inches of gravel on all sides for proper drainage.

Skipping the Sand Layer

Without sand, topsoil washes directly into gravel, reducing drainage capacity over time.

Cost Estimation and Budgeting

Material costs vary by region and supplier, but typical pricing ranges are:

Drainage gravel: $30-$50 per ton or $40-$60 per cubic yard
Coarse sand: $25-$40 per ton or $30-$50 per cubic yard
Perforated pipe: $0.50-$1.50 per linear foot (4-inch diameter)
Filter fabric: $0.10-$0.30 per square foot

For a 50-foot French drain, expect material costs between $300-$600 for DIY installation. Professional installation adds $1,500-$3,000 in labor. Use our gravel stone calculator to estimate your specific project costs.

Pro Tips for Successful Installation

•Maintain proper slope: French drains need 1% grade minimum (1 inch drop per 10 feet) for gravity drainage
•Install cleanouts: Add access points every 50 feet for future maintenance and inspection
•Use quality fabric: Non-woven geotextile fabric (4-6 oz) provides best filtration without restricting water flow
•Plan discharge location: Ensure water exits to daylight, dry well, or storm drain - never toward foundations
•Order 10% extra materials: Account for waste, spillage, and measurement variations

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How to Calculate Gravel and Sand Needed for a French Drain