Drainage gradient conventions
A falling run has an upstream invert higher than its downstream invert. For a ratio of 1:n, the vertical fall is the horizontal length divided by n. A 1:80 gradient therefore falls 12.5 mm per horizontal metre.
ratio denominator = horizontal length ÷ fall
How to calculate drainage fall
For a specified 1:n pipe gradient, divide the horizontal plan length by n to find the vertical fall, then subtract that fall from the upstream invert. When both invert levels are known, subtract downstream from upstream first, then divide the horizontal length by that fall to obtain the ratio denominator.
Worked drainage fall examples
1 in 40 fall over 12 metres
A 1 in 40 (1:40) fall is exactly 25 mm per horizontal metre. Over a 12.000 m plan length, the fall is 12 ÷ 40 = 0.300 m (300 mm). With an upstream invert of 100.500 m, the calculated downstream invert is 100.200 m. Practically, the invert reduces by 0.025 m for every metre travelled downstream.
Calculate gradient from two invert levels
For an upstream invert of 145.750 m, a downstream invert of 145.510 m and an 18.000 m horizontal run, the fall is 0.240 m. The ratio is 18 ÷ 0.240 = 1:75, equivalent to approximately 13.3333 mm/m, 1.3333% and 0.7639°.
These geometric checks assume one level datum and horizontal plan length. Use unrounded values for setting-out levels, follow the project’s stated precision, and verify the result against the approved drainage design.
For common ratios, see the gradient and fall reference table.
To check several connected runs, coordinate distances and repeated manhole levels together, use the Manhole Schedule Gradient Checker.
Manhole depth
When a ground level is supplied, the page calculates depth as ground level minus invert level. This is a simple level difference and does not add pipe wall thickness, bedding, cover requirements or chamber construction allowances.
Plan length and slope length
The formulas use horizontal plan length, which is the normal convention for gradient. For ordinary drainage gradients, the difference between plan and slope length is small, but project conventions should still be followed.
Frequently asked questions
What does 1:80 mean?
It means one unit of vertical fall for every 80 units of horizontal run. That is 12.5 mm per metre or 1.25 percent.
Why does the result say rising?
The downstream invert entered is higher than the upstream invert. Check that the flow direction and level order are correct.
Does the depth equal pipe cover?
Not necessarily. Ground-to-invert depth is not the same as cover to the outside crown of a pipe. Pipe diameter and wall thickness must be considered.