Free Drain Pipe Slope calculator with clear step-by-step results.
Converts a drainage slope into the total fall you have to build in, expressed three ways because plumbing codes use all three - percentage, millimetres per metre and one-in-x. The check compares your slope against the usual minimum for that diameter, since larger pipes need less fall to stay self-cleansing.
Total fall
Fall mm = run length m x slope x 1000
Gradient as a ratio
1 in x, where x = 1 / slope as a decimal
Drainage gradients, ventilation and pipe sizing are governed by plumbing codes that vary by jurisdiction. Work should be designed and inspected to the applicable standard.
Yes. Above roughly 5% the water can outrun the solids, leaving them behind to build up. Steep drops are better handled with a vertical stack than a long steep run.
Self-cleansing depends on flow velocity, and a larger pipe reaches the same velocity at a shallower gradient for a given flow. Oversizing a lightly loaded drain is a common cause of blockages.