Drainage Fall Gradient Calculator

About this calculator

Drainage pipes are not laid flat — they need a controlled fall so waste flows by gravity without solids settling out (too shallow a fall) or liquid outrunning solids (too steep a fall), and that fall is specified as a ratio like 1-in-40, meaning the pipe drops 1 unit of height for every 40 units it runs horizontally. The fall in millimeters over a given run is simply the run length divided by that gradient number.

Plumbers and groundworkers use this calculation to set invert levels correctly when laying foul or surface water drainage, since building regulations in most countries specify acceptable gradient ranges by pipe diameter and flow type — commonly around 1-in-40 to 1-in-110 for domestic foul drainage — and getting the fall wrong on a buried pipe is expensive to fix after the trench is backfilled.

This calculator takes your drain run length and required gradient and returns the exact fall in millimeters, so you can set your levels correctly the first time.