The minimum sewer pipe drain slope is directly correlated to the diameter of the pipe in inches. For example, a pipe diameter of 4 inches would have a minimum sewer pipe drain slope of 0.0084.

The calculation used to calculate the slope of the sewer pipe drain is S={V/[1.486/n*(D/4)^(2/3)]}^2. Slope is useful in draining sewer pipes because is is calculated to provide a minimum cleansing velocity for when the pipe is at a full flow or half flow. The minimum velocity is typically two to three feet per second. For larger pipe diameters, other considerations, such as ground subsidence may be evaluated in the calculations.