French Drain Slope Calculator

Why slope matters for a french drain

A french drain is a passive system. There is no pump, no pressure, and no valve to force water anywhere. All the water movement comes from gravity, which means the pipe has to run downhill for the whole length of the trench. If the slope is too shallow, water sits in the pipe, silt settles out, and within a few seasons the drain stops working. This calculator turns a length and a drop measurement into the two numbers that matter: slope percent and inches per foot.

The 1 percent rule

Most drainage references use 1 percent as the minimum acceptable slope for a residential french drain. That is 1 inch of drop for every 10 feet of run, or about 0.125 inches per foot. On a 40 foot run, that means the outlet needs to sit at least 4 inches lower than the inlet. Two percent (roughly 2 inches per 10 feet) is a more forgiving target and is what most installers actually aim for when the site allows it.

How to measure the drop

Stretch a mason line from a stake at the inlet to a stake at the outlet, level it with a line level, and measure the distance from the line down to the finished trench bottom at each end. The difference is your drop. A rotary or line laser is faster and more accurate on longer runs. Measure to the trench bottom, not the surface: rain does not care where the sod is.

What if there is not enough fall?

Deepen the trench at the outlet, move the outlet to a lower point on the property, or add a pop-up emitter, dry well, or catch basin further along the run to keep water moving. If none of that is possible, a passive french drain may not be the right tool and a sump pump or channel drain with a pumped discharge may be needed instead.