Drainage Slope and Fall Calculator
Works out the fall needed over a drain run.
Total fall
— mm
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
Slope — %
Fall per metre — mm/m
Depth at the bottom, on level ground — mm
The formula
fall = length ÷ gradient
A 1 in 40 gradient drops one unit for every 40 along.
What to watch out for
Too steep can be as bad as too shallow, because water outruns the solids.
Frequently Asked Questions
How much fall does a 10 m drain need at 1 in 40?
250 mm.