Drainage gradients are written as ratios - 1:40, 1:60, 1:80 - and the notation trips people up constantly on site. The rule: a 1:X gradient falls 1 unit vertically for every X units horizontally. So the bigger the number, the flatter the pipe.
The common gradients in mm per metre
| Gradient | Fall per metre | Fall over 5m | Fall over 10m |
|---|---|---|---|
| 1:40 | 25mm | 125mm | 250mm |
| 1:60 | 16.7mm | 83mm | 167mm |
| 1:80 | 12.5mm | 62.5mm | 125mm |
| 1:100 | 10mm | 50mm | 100mm |
The formula is: fall = run ÷ gradient denominator. A 7.2m run at 1:40 needs 7,200 ÷ 40 = 180mm of fall. Working backwards: if you have 150mm of available fall over 9m, your gradient is 9,000 ÷ 150 = 1:60.
Minimum falls for foul drainage
Standard guidance for 100mm foul drains works to 1:40 where flows are low, relaxing to around 1:80 where the run serves at least one WC (the flush helps keep the pipe self-cleansing). Flatter than that is designed territory - achievable, but justified by calculation, not rule of thumb. And steeper isn't automatically better: on very steep runs liquids can outrun solids, so long steep drains are engineered rather than laid to whatever the ground does.
Do the maths instantly
Our free drainage falls calculator converts between fall, run and gradient in any direction - built for setting out on site. For anything beyond a straightforward run - long, flat, deep, shared or adoptable - that's drainage design, and the calculator will tell you so honestly.
Frequently asked questions
What is a 1 in 40 fall in mm?
25mm per metre of run. Over a 6m drain, 150mm total fall.
What is the minimum gradient for foul drainage?
Rule of thumb from standard guidance: 1:40 for low-flow 100mm drains, down to about 1:80 with at least one WC connected. Flatter gradients need designed justification.
Is 1:80 okay for surface water?
Often, sized to the catchment - but surface water design is about capacity for storm flows, so pipe size and gradient are designed together rather than picked from a table.