A flow sensor is a device for sensing fluid flow. Typically a flow sensor is the sensing element used in a flow meter , or flow logger, to record the flow of fluids.

There are various kinds of flow meters, including some that have a vane that is pushed by the fluid, and can drive a rotary potentiometer , or similar device.

Other flow meters use a displacement piston, pushing it against a spring.

Flow meters are related to devices called velocimeters that measure velocity of fluids flowing through them. Laser-based interferometry is often used for air flow measurement, but for liquids, it is often easier to measure the flow. Another approach is Doppler-based methods for flow measurement. Hall effect sensors may also be used, on a flapper valve, or vane, to sense the position of the vane, as displaced by fluid flow.

A fluid dynamics problem is easily solved (especially in non-compressible fluids) by knowing the flow at all nodes in a network. Alternatively, pressure sensors can be placed at each node, and the fluid network can be solved by knowing the pressure at every node.

These two situations are analogous to knowing the voltages or knowing the currents at every node (noncompressible fluid being conserved in the same manner as Kirchoff's current or voltage laws, in which conservation of fluid is analogous to conservation of electrons in a circuit).

Flow meters generally cost more than pressure sensors, so it is often more economical to solve a fluid dynamics network monitoring problem by way of pressure sensors, than to use flow meters.