The ratios in plain numbers
A 1:40 gradient drops 25mm per metre - over a 10m run, 250mm. A 1:80 gradient drops 12.5mm per metre - 125mm over 10m. A 1:60 gradient drops roughly 16.7mm per metre. The arithmetic is simple: fall = distance ÷ gradient denominator.
For 100mm foul drains, standard guidance is a gradient of 1:40 where flows are low, and as flat as 1:80 where at least one WC discharges into the run and flows help keep the pipe self-cleansing. Surface water drains are commonly designed around similar gradients, sized to the catchment.
Too steep is also a problem
Intuition says steeper is better; drainage says otherwise. Very steep foul drains can let liquid outrun solids, leaving deposits behind. Long steep runs are normally broken with backdrop manholes or designed properly rather than laid at whatever slope the ground happens to have.
Use the calculator - then know its limits
Our drainage falls calculator converts between fall, distance and gradient instantly, which is exactly what you need when setting out on site. What a calculator can't do is design a system: minimum gradients depend on flows, pipe size and material, and adoptable sewers have their own standards. When the run is long, flat, deep or shared, that's drainage design - and we do it every week.
Frequently asked questions
What is the minimum fall for a 100mm foul drain?
As a rule of thumb from standard guidance: 1:40 for low flows, or down to about 1:80 where the run serves at least one WC. Flatter gradients are achievable with designed systems, but that's an engineering decision, not a default.
What is a 1 in 40 fall in mm?
25mm of drop per metre of pipe run. Over 6 metres, that's 150mm.