In this chapter we will investigate Venturi meters used in pipes as flow meters for incompressible fluids. A Venturi meter is a gage to measure the volume flow rate of a fluid in a pipe. Venturi meters have been used for water and waste water volume flow rate measurements for centuries. These gages use a converging and diverging nozzle connected in-line to a pipe. For measurement of the pressure drop in the converging nozzle, the ends of a U-tube that is partly filled with a measurement fluid of known density higher than that of water are attached to the upstream of the converging nozzle and to the throat area of the nozzle. The velocity of the fluid increases through the converging section of the nozzle, which relates to a corresponding decrease in the fluid pressure that is represented by the Bernoulli principle as given in Eq. (6.1) without any potential energy change between position 1 at the upstream of the converging nozzle and position 2 at the throat area of the nozzle. It is also assumed that the flow is steady, non-viscous, and irrotational.