Free French Drain Slope & Gravel Calculator
Calculate minimum trench slope elevation drop, washed drainage gravel tons & cubic yards, pipe sizing, and geotextile fabric wrap.
🌧️ Trench Dimensions & Slope
📊 Materials Takeoff & Elevation Drop
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
Why do the holes in a French drain pipe face down?
Perforations face down so that rising groundwater enters the pipe as soon as it reaches the bottom of the trench. If holes face up, water must fill the entire trench 6 inches deep before it can begin draining away.
What is the best gravel size for a French drain?
3/4-inch washed crushed stone or washed round river rock is the industry gold standard. It provides large interstitial void spaces (roughly 40% void ratio) for rapid water movement while preventing soil migration.
What type of fabric should I use to line a French drain?
Always use 4-ounce or 6-ounce non-woven needle-punched polypropylene geotextile fabric. Non-woven fabric allows high water flow-through (90-140 GPM/sq ft) while filtering out silt and clay particles.
How deep should a residential yard French drain be?
Typical depth is 18 to 24 inches. A minimum 2 inches of washed rock sits under the pipe, followed by the 4-inch pipe, 8 to 12 inches of gravel on top, the folded fabric, and 3 to 4 inches of topsoil or sod.