How to use it
- Enter the pipe length between the two manholes.
- Enter the slope as a percentage.
- Enter the upstream invert level, the inside bottom of the pipe where it leaves the upstream manhole.
- Enter the ground levels at both manholes to get the depth to invert.
Worked example
A pipe runs 40 m at 1% from an upstream invert of 98.500 m. The fall is 40 × 0.01 = 0.400 m, so the downstream invert is 98.100 m. With ground levels of 100.00 m and 99.80 m, the depth to invert is 1.50 m upstream and 1.70 m downstream.
Invert, cover and depth
The invert level is the inside bottom of the pipe. Depth to invert is measured from ground level down to that point, and it sets the manhole depth. Cover is the depth of soil above the top of the pipe, so it equals the depth to invert minus the pipe's outside diameter. Minimum cover protects the pipe from traffic loads and frost.
Frequently asked questions
How do I calculate the fall of a pipe?
Multiply the length by the slope. A 40 m pipe at 1% falls 0.40 m. At 1 in 80, it falls 40 ÷ 80 = 0.50 m.
What is an invert level?
The level of the inside bottom of a pipe or channel. Drainage drawings give invert levels at each manhole, and the slope is worked out between them.
How do I convert a slope ratio to a percentage?
Divide 100 by the second number. 1 in 40 is 2.5%, 1 in 100 is 1%, and 1 in 200 is 0.5%.
What minimum slope does a drain need?
It depends on the pipe size, the flow and your local rules. Small house drains are often laid at around 1 in 40 to 1 in 80. Use the gravity sewer calculator to check the velocity at your design flow.