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A flow meter is a device used to measure the flow rate or quantity of a gas or liquid in a pipe. Flow measurement applications are very diverse; consider these examples: water flow through an open channel, hydraulic valve leakage, fuel measurement through a fuel injector, or respiratory flow through a peak expiratory flow meter.

Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

Although there are many technologies used to measure flow, the positive displacement flow meter is unique as it is the only one to directly measure the actual volume. All other types infer the flow rate by making some other type of measurement and equating it to the flow rate. Flow meters are referred to by many names, such as flow gauge, flow indicator, liquid meter, etc. dependent on a particular industry; however the function, to measure flow, remains the same.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid's velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate.

The most common principals for fluid flow metering are: