Calculator for the calculation of flow velocity and flow rate of water

This Calculator serves the cal­cu­lation of average flow velocity and flow rate in rivers, creeks and canals when the cha­racteristic of the flow cross-section is known.


All input fields (white background) are filled out from the start, these values can be changed according to the in­di­vi­dual re­quire­ments.


The cal­cu­lated values are output in the green fields after a click on "Calculate" or pressing the enter button.


kSt   Slope %

Flow cross section

bo m       h m
bu m x m

Cross-sectional area
Wetted extent m
Flow velocity m/s
Flow rate m³/s

Background to this Calculator

The Calculation is based on the formula of Gauckler-Manning-Strickler.

Explanations concerning Strickler-coefficient kST

The Strickler-coefficient kST is a so-called roughness-coefficient. The larger this coefficient, the smoother is the channel bed and the lesser resistance is put against the flow. Therefore flow velocity increases with growing Strickler-coefficient.


Newly concreted channels have a very low roughness, whereas naturally or near-naturally rivers and creeks show a higher roughness, especially if they are characterised by dense bank vegetation and/or big stones.


In this calculator the Strickler-coefficient is output in the field "kST" after selecting the category. The value can be changed as required.

Explanations concerning slope

Slope expresses the proportion of height difference and distance. It is indicated in percent usually.


Example: 10 cm height difference over a distance of 10 m gives a slope of 0.1 m / 10 m * 100 = 1 %


Slope provides the energy for the motion of the water. Therfore flow velocity increases with growing slope.

Explanations concerning flow cross-section

Flow velocity also depends on the ratio of flow cross-section and wetted extent.


This Calculator allows the selection of simple flow cross-sections. Cross-sectional area and wetted extent are cal­cu­lated.


If you miss your desired flow cross-section in the drop-down menu, you have to choose "custom" and fill in values for cross-sectional area and wetted extent in order to calculate flow velocity and flow rate. The following figure shows what the term wetted extent means. It is plotted as a darkblue line:

Explanations concerning the calculated average flow velocity and flow rate

The cal­cu­lated values for average flow velocity and flow rate are ap­proxi­mate values. De­via­tions from the true values are possible. In case of greater water depths the cal­cu­lated values may be to high, especially if slope is large at the same time.

Further calculation tool

Related site

last modification: march 2018