The speed of a fluid changes as the cross-sectional area of the pipe through which it flow changes. The equation of continuity states that the volume of an ideal fluid flowing through a pipe is constant. This means that since liquids are incompressible the mass of fluid that flows through a pipe of different cross-section is also constant.

The product Av represents the volume of fluid per second that passes through the tube and is referred to as the volume flow rate. This means a fluid flows faster as the cross-sectional area of the pipe through which it flow decreases.