Work out drainage slope instantly with clear inputs, formula shown and shareable results.
Drainage slope is fall divided by length, expressed either as a percentage or the traditional one-in-N ratio. Velocity follows from Manning's equation with a hydraulic radius of one quarter the diameter for a half-full pipe, and the self-cleansing minimum is generally taken as 0.75 m/s.
Slope
Slope = fall / length; gradient = 1 in (length / fall)
Velocity
V = (1/n) x R^(2/3) x S^(1/2), with R = D/4 half full
Slope = fall / length; gradient = 1 in (length / fall). Drainage slope is fall divided by length, expressed either as a percentage or the traditional one-in-N ratio.
One in 80 for a 100 mm foul drain and one in 40 for smaller branches, but the real criterion is achieving self-cleansing velocity at the expected flow, not the gradient itself.
This calculator takes 3 inputs: Pipe run length, Total fall, Pipe diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.