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Estimates for steady, uniform gravity flow in US customary units. Verify against a full hydraulic analysis before design.
Manning's equation is an empirical formula used to estimate the flow of water in open channels and gravity pipes, such as ditches, culverts, storm drains, and sewer collection mains. It relates how fast water moves to the shape of the channel, how steep it is, and how rough its surface is.
It applies to steady, uniform, gravity-driven flow. It is not used for pressurized pipes, where the water fills the pipe and is pushed under pressure. For those, you size pumps and pressure mains instead.
Most of a utility's collection network moves water by gravity, not pressure. Sanitary sewers, storm drains, culverts, and open ditches all rely on slope and channel shape to carry flow. Manning's equation is how engineers check whether a channel or pipe has the capacity to carry the expected flow without backing up or overflowing.
Getting this right is a capacity and compliance question. An undersized storm drain floods streets. A sewer main laid at too flat a slope lets solids settle and clog. The same channel and asset data these calculations depend on lives in the systems a utility uses to manage its network, which is why connected water utility management software keeps pipe, slope, and material records in one place instead of scattered drawings.
The calculator uses Manning's equation in US customary units, where the constant 1.49 converts the units to feet and seconds. Flow rate is velocity multiplied by the flow area.
The calculator works out the area and wetted perimeter for you from the channel shape and depth, then solves for velocity and flow rate.
For a rectangular concrete channel 4 feet wide carrying water 2 feet deep, with a slope of 0.005 and n of 0.013:
Not sure which roughness value to use?
Each input shapes the result, so it helps to know what each one represents:
Three errors are common. The first is entering slope as a percent instead of a decimal, which overstates the flow by more than ten times. The second is applying the equation to a pressurized pipe, where the water is under pressure rather than flowing by gravity. The third is using a roughness value that does not match the real condition of the channel, since a rough, debris-filled pipe carries far less than a clean one.
It estimates the velocity and flow rate of water in open channels and partially full pipes that flow by gravity, such as storm drains, sewers, culverts, and ditches. Engineers use it to check whether a channel has enough capacity for the expected flow.
It uses US customary units. Enter dimensions in feet and slope as a decimal. Results are given as velocity in feet per second and flow rate in cubic feet per second. The formula uses the 1.49 constant for these units.
Match the value to the channel material and its condition. Smooth plastic pipe is around 0.011, concrete around 0.013, corrugated metal around 0.024, and natural earth channels range from about 0.022 to 0.035. Use a higher value for older or debris-filled channels.
These related calculators cover the rest of a water and wastewater system:
To see how one platform keeps the pipe, slope, and asset records behind these calculations in a single place, explore SMART360 water utility management software.
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