French Drain Calculator
Plan your French drain project with accurate material estimates. Enter trench length, width, depth, and pipe size to get gravel volume in cubic yards and tons, pipe length, filter fabric requirements, and flow capacity. Optional pricing fields give you a complete project cost estimate. Whether you are diverting water from your foundation or draining a wet yard, this calculator has you covered.
Total length of the French drain trench
Standard French drain trench is 12 inches wide
Deeper trenches handle more water; 18 in is standard
Filter fabric prevents soil from clogging the gravel
Affects gravel compaction and drainage performance
Optional — cost per linear foot of filter fabric
Optional — cost per ton of drainage gravel
How much gravel do I need for a French drain?
Gravel volume = trench volume minus pipe volume. A standard 50-ft trench, 12 in wide × 18 in deep: Trench volume = 50 × 1 × 1.5 = 75 cu ft = 2.78 cu yd. Pipe (4 in diameter) displaces about 0.44 cu ft per ft = 22 cu ft total. Net gravel = 75 - 22 = 53 cu ft = 1.96 cu yd ≈ 2.75 tons. Use 1.5-inch washed gravel (No. 57) for best drainage. Rounded gravel drains better than crushed angular stone.
How deep should a French drain be?
Standard depth is 18-24 inches. For surface water near a house: 18 inches minimum. For foundation drainage to redirect groundwater: 24-36 inches deep, sloping away from the foundation. The drain should be below the problem water level and above any utility lines. Always maintain 1/8 inch per foot slope (1% grade) toward the discharge point. Check local codes — some areas require permits for deep drainage systems.
Do I need filter fabric around my French drain?
Yes, filter fabric is strongly recommended. It prevents soil particles from migrating into the gravel and clogging the drain. Without fabric, clay and silt will fill the voids between gravel within 1-3 years, rendering the drain useless. Fabric-wrapped drains last 20-30 years versus 5-10 years without. Wrap both the pipe and the gravel bed — leave excess fabric to fold over the top before backfilling with soil.
What size pipe should I use for a French drain?
4-inch perforated pipe is standard for most residential French drains. Use 3-inch for small areas like a single downspout. Use 6-inch for large commercial systems or areas with heavy runoff. The pipe should be Schedule 40 PVC or HDPE (corrugated) with 360° perforations. Place the pipe with holes facing down to allow water entry from below. Wrap the pipe in filter fabric or a sock to prevent gravel intrusion.