Droplet Logo
Davenport Flow Technology Logo

Simple Flow Calculations


Open Channel Flow - Velocity Flow

Nearly all flow measurement systems, techniques and calculations –
make use of the basic fact that Q = V A

  Start Calculator

In a round pipe (as long as it is running full) the cross sectional area is dependent only on the pipe diameter and the basic equation can be written as:-


Q = V π D2

Where: D =

the inside diameter

of the pipe.


In open channels or pipes which are not running full the cross sectional area will change if the depth of increases or decreases.
For a rectangular channel the basic equation becomes:-


Q = V B h

Where: B =

h =

the width of the channel
the depth of the flow.


“The rate of flow of a liquid moving in a pipe or an open channel is dependent on the velocity and the cross sectional area of the liquid”.  

The basic equation for most flow measurement applications is:-

Q = V A



   Q = Flow rate e.g. m3/s

   V = Velocity e.g. m/s

   A = Area e.g. m3/s

“An Open Channel is any pipe or conduit where the water surface is at atmospheric pressure”

The term “Velocity Area” usually refers to measuring flow in an open channel or partly filled pipe where the flow velocity and cross sectional area are measured separately.

In a channel with a regular cross section, the area is usually obtained by measuring the depth of flow (the head)


The simplest method of measuring flow in an open channel is to measure the depth of flow and calculate the cross sectional area and then to estimate the velocity by throwing a stick or other floating object into the flow stream and measuring the time it takes to travel a set distance!

It is not a very sophisticated technique but can often be a first step before deciding on more sophisticated systems.


The following flow calculations are available free of charge:-


Open channel Slope Area Method (Manning Equation)

Open channel Flumes

Open channel V notch