Sometimes we need to know the volume of water that is passing through a hose. If you are going to fill up a very large tank of water or even a swimming pool then you may want to know how long it might take. Instead of having to calculate this manually you can rely on programs that can either be downloaded from the internet or use an online calculator on this flow meter portal.
If you know the internal diameter of the water hose and the velocity/speed of the water flowing, then you can easily find out the volumetric flow in gallons, meter cubed or other metric or imperial units. This calculator was designed for volumetric flow and not mass flow. Using it for water will be fine.
If you have no idea how to calculate the velocity it is quite simple. You can measure the length of the hose when empty and after you have hooked it up to the water supply you then use a stop watch to see how much time it takes for the water to reach the hose outlet e.g if your water hose is 30ft long and you see that it takes 30 seconds to see water fill up and reach the empty hose end after turning on the water supply, then your flow velocity will be 30 ft divided by 30 seconds i.e. 1ft/second.
The volume calculation uses the internal surface area of the water hose coupled with the velocity to produce the volume flow rate which is what you are actually looking for.
If you are dealing with a hydrocarbon it will be another matter altogether because the calculation will need to consider such things as changes in temperature and pressure and you may want to calculate mass flow instead.