French Drain Gravel Calculator - Free 2026

For a standard residential French drain, you need gravel to fill the trench around the perforated pipe. A typical 12-inch wide by 18-inch deep trench requires approximately 0.67 cubic yards of gravel per 10 feet of length. The calculation accounts for gravel below the pipe (4 inches), around the pipe (4-inch diameter), and above the pipe (8 inches), with a 10% waste factor for settling and spillage.

Example calculation: 50-foot drain × 12" wide × 18" deep = 75 cubic feet = 2.78 cubic yards base + 0.28 cubic yards waste = 3.06 cubic yards total. Always round up when ordering to avoid shortages during installation.

The best gravel for French drains is 3/4-inch to 1-inch clean, washed angular gravel (also called drainage stone or drain rock). This size provides optimal drainage while preventing the perforated pipe holes from clogging. Avoid pea gravel (too small) and large river rock (too big). The gravel should be clean and free of fines (sand and dust) to maintain high permeability and prevent clogging over time.

Clean angular stones create void spaces of 35-40% for water flow, while maintaining structural stability. Round river rock compacts less effectively, and pea gravel blocks water flow when it settles. Always specify "clean washed drainage stone" when ordering from suppliers.

Yes, filter fabric (landscape fabric or geotextile) is essential for French drains. It wraps around the gravel to prevent soil from infiltrating and clogging the drainage system. Use non-woven geotextile fabric rated for drainage applications. The fabric should line the entire trench before adding gravel, then wrap over the top after installation. This extends the drain's lifespan from 10-15 years to 30-50 years by preventing sediment buildup.

Choose 4-6 oz per square yard non-woven geotextile fabric. Woven landscape fabric restricts water flow and fails prematurely. Line the trench completely, overlap seams by 6 inches, and fold over the top with 6-inch overlap for complete protection. Related drainage projects benefit from similar material planning - check our subbase calculator for foundation work.

Standard residential French drains are typically 18-24 inches deep. The depth depends on the water source and discharge point. For foundation drainage, dig 6 inches below the bottom of the footing. For surface water management, 12-18 inches is sufficient. The trench should slope at least 1% grade (1 inch drop per 8 feet) for proper drainage. Deeper installations (24-36 inches) are needed for high water tables or commercial applications.

Depth also affects excavation difficulty and cost. Hand digging works for 18-inch depth in soft soil. Deeper trenches or hard soil require mechanical trenchers ($75-$150/day rental) or excavators ($200-$400/day rental). Always call 811 before digging to locate underground utilities.

French drain installation costs $25-$50 per linear foot for professional installation in 2026, or $8-$15 per foot for DIY projects. A typical 50-foot residential French drain costs $1,250-$2,500 installed, or $400-$750 for materials only. Costs include perforated pipe ($1-$2/ft), drainage gravel ($40-$60/cubic yard), filter fabric ($0.50-$1/sq yd), and excavation labor ($15-$25/ft). Additional costs apply for obstacles, deep excavation, or connecting to municipal storm drains.

DIY savings reach 60-75% but require physical labor, tool rental, and 2-3 days' time. Professional installation includes trenching equipment, proper grading, disposal of excavated soil, and warranty on workmanship. Complex layouts or connection to existing drainage systems justify professional installation costs.

No, you should not use regular gravel or pea gravel for French drains. Drainage gravel (3/4-inch angular stone) is specifically washed and sized for optimal water flow. Regular gravel contains fines (sand and dust) that clog the system and reduce drainage capacity by 60-80%. Pea gravel is too small and round, causing it to compact and block water flow. Using proper drainage gravel ensures your French drain functions effectively for 30-50 years instead of failing within 5-10 years.

The $5-$10 per cubic yard premium for clean drainage stone versus regular gravel is worthwhile insurance. Failed drainage systems require complete excavation and replacement at 3-5 times the original material cost. For projects requiring backfill around drainage systems, consult our backfill calculator for proper material selection.

Perforated French drain pipe has holes or slots spaced 6-12 inches apart around the circumference. You don't drill these yourself - purchase pre-perforated 4-inch corrugated or PVC pipe from hardware stores. Install the pipe with holes facing down (not up) to prevent sediment from entering. The pipe sits on 4 inches of gravel, surrounded by 4-6 inches of gravel on all sides, then wrapped with filter fabric to keep the system clean.

Corrugated pipe costs $0.75-$1.25 per foot with perforations every 6-8 inches. PVC Schedule 40 perforated pipe costs $1.50-$2.50 per foot with slots every 4-6 inches. PVC offers smoother interior for better flow and 50+ year lifespan versus 30-40 years for corrugated.

The minimum slope for a French drain is 1% grade, which equals 1 inch of drop per 8 feet of length (or 1 foot drop per 100 feet). This ensures gravity-fed drainage without standing water. For better performance, aim for 2% grade (1 inch per 4 feet). Mark the slope with string lines and a level before digging. The discharge end must be lower than the collection end, with the outlet above ground to prevent backflow during heavy rain.

Use a 4-foot level or transit laser to check grade every 10 feet during excavation. Place stakes with marked elevations at the start and end points. Insufficient slope causes water to pool in the pipe, reducing capacity and promoting sediment accumulation. Excessive slope (over 5%) can cause erosion and requires pipe anchoring every 10 feet.