To calculate the water flow rate, square the pipe's diameter, and then multiply that number by pi, .25 and the velocity of the water flow. For example, a pipe with a diameter of 3 inches, or .25 feet, and a water velocity of 20 feet per minute yields a flow rate of approximately 0.98 cubic feet per minute. Convert this number to the unit of measure for residential flow rates to yield a final answer of 7.34 gallons per minute.
Building inspectors can calculate the flow rate of an individual faucet empirically using a 5-gallon bucket and a stopwatch. They place the bucket underneath the faucet and time how long the faucet takes to fill the bucket completely with water. Then they can divide the capacity of the bucket by the number of minutes and seconds it takes to fill the bucket. For example, if it takes two minutes to fill the 5-gallon bucket, then faucet has a flow rate of 2.5 gallons per minute.
A water flow rate calculated using the pipe's diameter and water velocity does not take into account the length of the pipe, friction within the line, bends in the plumbing and other circumstances that affects flow rates.
A typical water flow rate is 1 to 3 gallons per minute for a bathroom faucet, 2 to 4 gallons per minute for a kitchen faucet and 2 to 6 gallons per minute for a showerhead.