How to Calculate GPM in Pipe

By Marco LoPresti
With enough information, the flow can be calculated.

When working with pipes, it's important to know the flow rate to confirm that the pipe meets requirements, or to know how much fluid is being supplied. Flow rate is measured by gallons per minute, or GPM. Given key information about the piping system, the flow rate can be calculated using the appropriate engineering formulas. The general formulas are quite involved and require some technical expertise to use. However, for specific cases, the formulas can be greatly simplified.

Get the details for the piping system and fluid. For example: pipe material is plastic; length is 200 feet; diameter is 3/4 inches; inlet pressure is 30 pounds-per-square-inch (PSI); outlet pressure is 20 PSI; fluid is water at room temperature (approximately 68F).

Use the correct formula. For this example, a formula for a good approximation is: GPM = 3.0877 x p^0.5415, where "P" represents the pressure drop in PSI per 100 feet of pipe. The ^0.5415 represents a function on a scientific calculator which raises a number to a power. The button to is usually indicated by "Y" with a small raised "X," or vice versa.

Carry out the calculations. The total pressure drop is 30 PSI minus 20 PSI, which equals 10 PSI for 200 feet of pipe. The formula requires pressure drop per 100 feet. To get this, divide the total pressure drop by the pipe length and multiply by 100. In this case, it is 10 divided by 200 times 100, which equals 5 PSI. To get the flow, multiply 3.0877 by 5 raised to the power of .5415, which equals 7.38. The flow is 7.38 GPM.

Tip

You can look up flow rates for other pipe sizes and pressure drops at sites like plumbingsupply.com. Note, the pressure drop in some references is expressed as "feet of head." To convert to PSI, multiply by 0.434.

Warning

The formula given in this article applies only to the case described and for a pressure drop per 100 feet of pipe in the range of 2 to 30 PSI. Other fluids, temperatures, pipe materials, pipe sizes and pressure drops would require different equations. If fittings are in the pipe, these have to be treated specially. The pipe is assumed to be horizontal. If one end is higher than the other, the formula needs to be adjusted to account for this.

About the Author

Marco LoPresti has been employed as a professional engineer in the energy field for over 30 years and has written many technical reports and presentations during his career. He holds a Bachelor of Science from Dalhousie University and a Bachelor of Engineering (Mechanical) from DalTech. He lives in Nova Scotia, Canada.