How the maths works
A gradient of 1:X means the pipe drops 1 unit vertically for every X units horizontally. Three rearrangements of the same formula cover every site situation:
- Fall = run ÷ X - a 10m run at 1:40 needs 10,000 ÷ 40 = 250mm of fall
- Gradient = run ÷ fall - 150mm of fall over 9m is 9,000 ÷ 150 = 1:60
- Maximum run = fall × X - 200mm of available fall at 1:80 allows a 16m run
Quick references: a 1:40 gradient means a 25mm fall per metre; a 1:80 gradient means a 12.5mm fall per metre; 1:60 is 16.7mm and 1:100 is 10mm per metre.
When a calculator isn’t enough
Falls are one variable in drainage that works: pipe size, flows, self-cleansing velocity, depth, cover, bedding and what the run connects into all matter - and adoptable sewers are designed to water company standards, not rules of thumb. If your run is long, flat, deep, shared or heading for adoption, that’s drainage design - exactly what our civil engineering team does. For the background reading, see our guide to drainage falls and gradients.
Frequently asked questions
What is a 1 in 40 fall per metre?
25mm per metre. Divide 1,000mm by the gradient denominator: 1,000 ÷ 40 = 25mm of fall for every metre of pipe run.
What is a 1 in 80 fall in mm?
12.5mm per metre of run - half the fall of 1:40. Over a 10m run, that's 125mm.
How do I calculate a drainage fall?
Total fall = run ÷ gradient denominator (in consistent units). For a 12m run at 1:60: 12,000mm ÷ 60 = 200mm of fall.
What fall should a foul drain have?
Rule-of-thumb guidance for 100mm foul drains is 1:40 for low flows, relaxing to about 1:80 where at least one WC discharges through the run. Outside that range - or on adoptable, deep or shared drains - the gradient should come from a drainage design, not a table.