Continuity equation formula: S1V1=S2V2. The continuity equation is the specific expression form of the law of conservation of mass in fluid mechanics. Its premise is to adopt a continuous medium model for fluids. Velocity and density are continuous and differentiable functions of spatial coordinates and time.

In physics, the continuity equation is a partial differential equation that describes the transmission behavior of conserved quantities. Since mass, energy, momentum, charge, etc. are all conserved quantities under their respective appropriate conditions, many types of transport behaviors can be described by continuity equations. The continuity equation is a local conservation law equation. Compared with the global conservation law, this conservation law is a stronger version. All examples of continuity equations in this article express the same idea. The change in the total amount of a conserved quantity in any region is equal to the amount entering or leaving from the boundary; the conserved quantity cannot increase or decrease, only Ability to move from one location to another.