A Venturi is a system for speeding up the flow of fluid by constricting it in a cone-shaped tube, which causes the fluid to increase its velocity and reduce its pressure, producing a partial vacuum. The Venturi effect is named after its discoverer, the 18th-century Italian physicist Giovanni Battista Venturi. The Venturi effect is utilized in many different devices, including vacuum cleaners, ventilators, and diffusers in automobiles.

Venturi tubes are the simplest apparatus used to apply the Venturi effect. They are tubular setups with a length of pipe of varying diameter, and they typically have an entry cone of 30 degrees and an exit cone of 5 degrees to avoid undue aerodynamic drag. Venturi tubes are often used in processes where permanent pressure loss is not tolerable and where maximum accuracy is needed in case of highly viscous liquids.

Venturi tubes and orifice plates are used in industrial applications and scientific laboratories for measuring the flow rate of liquids. A Venturi can be used to measure the volumetric flow rate using Bernoullis principle. By measuring the change in pressure, the flow rate can be determined by using a Venturi tube. Manometers or transducers are installed on the two different sections ("free flow" and small) to measure the pressure difference. Since we know the geometry of the Venturi meter (A1, A2), and the fluid itself (viscosity and density), the flow rate can be easily calculated.

Venturi tubes are more expensive to construct than simple orifice plates, and both function on the same basic principle. However, for any given differential pressure, orifice plates cause significantly more permanent energy loss. Venturi injectors have no moving parts and are maintenance-free, and they operate effectively over a wide range of pressures and flows.