French Drain Slope Calculator
Calculate exact gravity pitch drops, starting and ending trench depths, and aggregate washed gravel tonnage for yard drainage systems.
Recommended Tools & Equipment
Tested hardware and components for high reliability
How to Pitch a French Drain
A French drain relies completely on gravity to move groundwater away from foundations and waterlogged yards. The industry standard slope is 1/4 inch of vertical drop for every 1 foot of horizontal run (approx 2% grade).
Total Elevation Drop Formula
Drop (inches) = Run (feet) × Pitch per foot
Example: A 50-foot run at 1/4" per foot requires 12.5 inches of total fall.
Why Washed Round Stone is Critical
Never use crush-and-run, limestone screenings, or unwashed pea gravel. Fine stone dust compacts into a cement-like barrier that blocks water entry. Always specify 3/4" to 1-1/2" washed round river rock or crushed granite wrapped completely in non-woven needle-punched geotextile fabric.
Frequently Asked Questions
Should the holes in the perforated pipe face up or down?
The holes MUST face DOWN. Groundwater enters from the bottom of the gravel trench as the water table rises. Facing the holes down allows water to enter the pipe at the lowest possible level and flow out immediately.
Can a French drain be installed flat if there is no slope?
A completely flat French drain will act as a dry well storage trench, but water will sit stagnant rather than flowing freely away. If your yard has zero grade, you must dig the trench progressively deeper toward the exit point.
Why should I use non-woven geotextile fabric?
Non-woven fabric allows water to pass through at high flow rates while physically blocking clay and silt particles from infiltrating the gravel and clogging the drainage pipe pores over time.