Manning's equation is an empirical formula for calculating open channel flow. Open channel flow is free-surface flow driven by gravity. We have added a simple, user friendly manning's equation calculator to this site available. You can also access the manning's equation calculator by clicking on the image shown.

Manning's equation was introduced in 1889 by Robert Manning (Irish Engineer) and it is used as an alternative to the Chezy equation. The variables in Manning's equation are explained as follows:

Flow (Q): The flow rate in cubic-feet per second (ft³/s) is the flow that is calculated for the channel or conduit in question.

Area (A): The area A in square feet (ft²) is the cross-sectional area or the flow area. If the channel is partially full this area will be different than the area of the channel.

Manning's coefficient (n): Manning's roughness coefficient represents the friction fromt the channel or the conduit. We have provided some Manning's n roughness coefficients on this site. Select the n-value cautiously since you selection can greatly affect your results.

Hydraulic radius (R): The hydraulic radius in feet (ft) is the ratio of the flow cross-sectional area to the wetted perimeter. That is R = A/P where P is the wetted perimeter.

Slope (s): The unitless slope s is the longitudinal slope of the channel or conduit in the direction of flow