Flow Rate

Flow rate can be defined as the speed or rapidness of a fluid or gas that flows through a channel or a vessel. This rate is a mean of measuring the speed or velocity of liquid or gas that is meant to flow from a particular channel, pipe or a vessel.

The flow rate within the pipe is defined as the volume of fluid that is passing through a cross-sectional slice of the pipe each second. It turns out that under the assumption of uniform flow within the pipe, the flow rate, which is often represented by the symbol Q, can be directly related to the fluid speed v and a measure of the pipe's cross-sectional area, or A. In terms of these variables, the flow rate Q can be expressed as:

                                                                                                                    Q = V . A

where the units of Q are volume per time, or cubic meters per second in Standard International (SI) units. When applying this equation, your answer will only be in SI units if your area and speed are expressed in SI units: square meters and meters per second, respectively.

As this formula expresses, the flow rate in a pipe directly relates to fluid velocity and also the pipe's cross-sectional area. In other words, the higher the fluid velocity or cross-sectional area, the higher the flow rate.

A pipe carrying fluid can be in any shape in general, but the meaning of A in the formula is always the same - it is the area of a slice that the fluid is flowing through. It is important to remember the reason why flow rate depends on A: wider pipes can deliver larger volumes of fluid at a higher rate than narrower pipes.