A fluid passing through smoothly varying constrictions is subject to changes in velocity and pressure, as described by Bernoulli's principle. A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone-shaped tube. They are found in many applications where the speed of the fluid is important, and form the basis of devices like a carburetor.

Named after Giovanni Battista Venturi (1746–1822), Italian physicist.

Venturis are also used to measure the speed of a fluid, by measuring pressure changes from one point to another along the venturi. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a venturi is all that is needed. When the fluid flows though the venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.

Venturi are more expensive to construct than a simple orifice plate which uses the same principle as a Venturi, but the orifice plate causes significantly more permanent energy loss and is less accurate.

A venturi can also be used to mix a fluid with air. If a pump forces the fluid through a tube connected to a system consisting a venturi to increase the water speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), air will be sucked in trough the small hole because of changes in pressure, and at the end of the system a mixture of fluid and air will appear.

To avoid undue drag, a venturi typically has an entry cone of 30 degrees and an exit cone of 5 degrees.