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How to Size a Submersible Well Pump

Cassandra Tribe

Submersible well pumps are seated beneath the water level in the well. These pumps are considered to last for a long time and mostly maintenance-free (unless there is a high level of sand and algae) and more efficient than non-submersible pumps with the same pump engine horsepower.

The diameter of the pipe effects the rate of water flow.

Sizing a submersible well pump requires knowledge of the well head distance, or the distance from the pump when submersed to the top of the well, the diameter of the water feed pipes, and the number of appliances connected to the system.

  1. Count the number of appliances in the household that use water. The most typical types of appliances are: shower/bathtub, lavatory (bathroom sink/bidet), kitchen/utility sink, clothes washer and dishwasher. Add any special appliances to the list, such as an indoor water sculpture, if necessary. Be sure to count all outside water spigots or permanent sprinkler systems as an appliance. The total number of appliances connected to the water system is equal to the total gallons of water per minute the well pump must be able to deliver.

  2. Write down the diameter of the well water pipe. The diameter can be found on the plat drawings registered with the local building authority, or a close approximation of the diameter can be found by measuring the exterior of the pipe at the well and subtracting 1/4-inch from that diameter. A close approximation is acceptable as the interior of the pipe may have narrowed from its diameter at installation due to algae or corrosion. Standard water pipe diameters are 4, 6, 8, 10 and 12 inches.

  3. Compare the maximum gallons per minute that can be pumped through the water pipe to the maximum gallons per minute needed by the system. The diameter of the pipe determines the GPM possible: a 2-inch diameter pipe can deliver up to 90 GPM, a 4-inch diameter pipe can deliver up to 400 GPM, a 6-inch diameter pipe can deliver up to 600 GPM, a 8-inch diameter pipe can deliver up to 90 GPM, a 10-inch diameter pipe can deliver up to 1,000 GPM, and a 12-inch diameter pipe can deliver up to 2,000 GPM,

  4. Sort the available submersible well pumps by the maximum GPM they can deliver. Make sure the maximum GMP meets the criteria of the GPM required by the number of appliances and the rate at which water may be pumped through the water pipe. Often, pumps are rated by gallons per hour (GPH) rather than minute. Either divide the GPH by 60 to find the pumps GPM rating, or multiply the appliance GPM by 60 to covert it to GPH. Select pumps are closest in maximum pumping ability to the GPM needed by the appliances, not the maximum GPM the water pipe is capable of carrying.

  5. Read the depth rating of the pump. The depth rating is also called the "head" or "head measurement." This is the distance between the submerged pump and the top of the well. The head measurement will impact the efficiency of the pump in regards to the pumping power (engine horsepower) being able to raise the water vertically to the water feed line. A higher horsepower engine will be needed for deeper head measurements.

  6. Calculate the projected efficiency of the pumps to help narrow the choices further. Divide the total by multiplying the GPM by the Head Measurement by 100 by the total found when multiplying the input horsepower of the pump engine by 3,960. The pump with the greatest projected efficiency that meets the required GPM needs of all the appliances without exceeding the maximum flow rate allowed by the water pipe diameter -- and that remains within the purchasing budget -- is the pump to choose.

  7. Tip

    Use a professional to install the pump and connect the water lines. While the job is something a do-it-yourselfer can accomplish, it is more advisable to have a licensed professional do the job to prevent mishap and to provide repair and maintenance services later.


    Bigger is not better. Do not purchase a pump that is larger than needed, or there is an increased risk of the pump drawing air and causing disruption to both the workings of the pump and the water flow through the pipes.