How to Size a Submersible Well Pump
Providing a reliable supply of fresh water to a home is the most important function of a private well system. Selecting the wrong pump—either one that can't move enough water or one that can't push against the required pressure—leads to low pressure in the shower, rapid wear on the motor, and potentially expensive replacements. This Well Pump Sizing Calculator helps you determine the two most critical specifications: Flow Rate (GPM) and Total Dynamic Head (TDH).
Calculating Flow Rate (GPM)
The flow rate is the volume of water the pump can deliver per minute. In residential applications, the industry standard for sizing is the one-for-one rule. For every water fixture in the house (faucets, toilets, showers, washing machines, outdoor spigots), you should provide 1 GPM of pump capacity. For a typical 3-bedroom home with 10 to 12 fixtures, a 10 to 12 GPM pump is recommended. This ensures that even if multiple people are using water at once, the pressure remains stable.
Understanding Total Dynamic Head (TDH)
The "Head" is a measurement of the resistance the pump must overcome to deliver water at the desired pressure. It is not just the depth of the well. TDH is the sum of:
- Static Head: The vertical distance from the water level in the well to the highest point of water use or the pressure tank.
- Pressure Head: The vertical lift equivalent to your desired PSI. Every 1 PSI requires 2.31 feet of lift. If you want 50 PSI, that adds 115.5 feet of head.
- Friction Loss: The energy lost as water rubs against the pipe walls. For standard residential systems, we add a 10% buffer to account for this.
Why TDH is the "Performance Killer"
A pump rated for 15 GPM does not provide 15 GPM under all conditions. As the "Head" (the vertical distance and pressure) increases, the pump's output decreases. If your well is 200 feet deep and you want 60 PSI, a pump rated for 15 GPM at 50 feet might only provide 5 GPM at your home. Always check the Pump Performance Curve to ensure the unit can deliver your required GPM at your calculated TDH.
Frequently Asked Questions
What happens if my well has a low yield?
If your well can only produce 2 GPM but your home needs 10 GPM, you cannot simply buy a larger pump. A large pump will "dry out" the well and burn itself out. In these cases, a "Holding Tank" or "Cistern" system is used, where a small pump slowly fills a large tank over 24 hours, and a separate booster pump provides the pressure for the home.
Should I get a 1/2 HP or 3/4 HP pump?
Horsepower (HP) is secondary to GPM and Head. A 1/2 HP pump with more stages can often push water higher than a 3/4 HP pump with fewer stages. Use our calculator to find your GPM and TDH requirements first, then select the motor that achieves those numbers on the performance chart.