The venturi tube calculation is an application of Theorem Bernoulli :
The equation Bernoulli can be regarded as a principle of conservation of energy suitable for moving fluids. The behavior usually called "Venturi effect" or "Bernoulli effect" is the reduction of fluid pressure in the regions where the velocity is increased. The decrease of pressure in a duct stricture may seem contradictory, unless we consider the pressure as a Energy density . By the way in narrowing , the fluid velocity, so the fluid kinetic energy, must increase at the expense of the fluid pressure energy.
Assuming that the flow is laminar , That friction pressure losses of pipe are negligible, and that the speed is average speed :
The window of Venturi tube in tools
This window allows calculation of venturi tube versus section, density, flow or pressure drop measured. Whether conducting a flow meter, a vacuum system ...