**To calculate the flow rate in a cylindrical pipe, the basic formula, which is flow rate is equivalent to pipe cross-sectional area multiplied by velocity, can be used.** Here the area is calculated as pi times pipe diameter squared and divided by four.

Flow rate, velocity and pipe cross-sectional area are typically symbolized by Q, v and A in a formula. The pi used for the pipe's area is taken to be 3.14. Units that are commonly used for the flow rate of water systems are gpm, or gallons per minute, cubic meters per second and liters per second. The SI unit for flow rate is cubic meters per second.