French Drain Slope Guide
Minimum and recommended slope for a french drain, how to calculate required drop over the run, and how to hit daylight at the outlet.
A french drain needs at least 1 percent slope, or 1 inch of drop per 10 feet of run. Steeper is fine. Flatter risks standing water inside the pipe.
Why 1 percent
Perforated pipe carries water by gravity. Below about 1 percent, the flow is too slow to keep sediment moving, so silt drops out and eventually plugs the pipe.
Required drop
- 1 percent minimum: 1 inch per 10 feet
- 2 percent recommended for foundation drains: 2 inches per 10 feet
- Long runs (over 50 feet): confirm the outlet elevation is below the far end of the trench bottom before digging
Hitting daylight
The pipe must exit to daylight (an open ditch, dry well, or storm inlet) that is lower than the trench bottom at the low end. If your yard grade does not allow that, you need a sump pump and solid discharge pipe, not a passive french drain.
Ready to run the numbers?
Use the french drain slope calculator to get a specific quantity for your project.
Open the french drain slope calculator →Related guides
Last reviewed . Educational estimates only; local conditions and codes take precedence. Consult a qualified professional for site-specific decisions.