Pumping Terminology: What Is GPM and How Is it Calculated?

what is gpm for a pump and how do you calculate it

When it comes to pumping terminology, one crucial term to know is GPM — a measurement that will help you determine if you’re choosing the right pump. So what is GPM, and how do you calculate it?  

What Is GPM?

GPM stands for gallons per minute and is a measurement of how many gallons a pump can move per minute. It is also referred to as flow rate. GPM is variable based on another measurement known as the Head, which refers to the height the water must reach to get pumped through the system. 

There are several different Head measurements, including:

  • Total Head: The discharge point for the fluid.
  • Total Suction Head: The height between the fluid source and the pump.
  • Total Discharge Head: The height from the pump to the highest point that the fluid will get pumped.

Pumps are typically measured by their GPM at a certain Head measurement. For example, a pump specification may read 150 GPM at 50 Feet of Head, which means the pump will work at 150 gallons per minute when pumping water at a height of 50 feet. 

How to Calculate GPM

The GPM formula is 60 divided by the number of seconds it takes to fill a one gallon container. So if you took 10 seconds to fill a gallon container, your GPM measurement would be 6 GPM (60/10 seconds = 6 GPM). To most accurately calculate GPM, you use the pressure tank method and formula. For this calculation, you need to know the specifications of your pressure tank, including how many gallons it holds, the gallon drawdown and the PSI. The manufacturer specifies the gallon drawdown. Once you have that information, as well as a stopwatch to keep time, follow these steps:

  • Empty the pressure tank
  • Start your stopwatch when you hear the pressure switch turn on
  • Stop the stopwatch when the switch turns off 
  • Divide the time on your stopwatch by the drawdown amount

For example, if it took four minutes for the pressure switch to turn off, and your gallon drawdown was 20 gallons, this would mean a GPM rate of five. 

If you don’t have a pressure tank, you can also use a bucket or any other container, time how long it takes to fill up and then divide that by the volume the container holds. 


GPM identifies the unique capabilities of a pump so you can select the right one for your specific needs. If you need a pump for a larger public area such as a golf course, marina or lake, you will need a pump with a much higher GPM than one used for your home’s well. Plus, choosing the correct pump is essential for reducing your costs and increasing your pump’s lifespan.

GeoForm International – A Leader In Pumping Technology

At GeoForm International, we are a leading manufacturer of high-quality submersible pumps, dredges, digester packages and aerators, all of which are made in the U.S. With our pump expertise, we know just how essential GPM is in the pumping and dredging industry from how much equipment costs to how long jobs will take. 

If you have any questions, contact us today to speak with one of our experts. 

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