How to Use the Pump Shaft Power Calculator
Before sizing a motor to drive a pump, you need to know the shaft power the pump actually requires. Enter flow rate (Q, GPM), head (H, ft), and pump efficiency (%), and this calculator applies the standard US customary pump horsepower formula used throughout the industry.
For water (specific gravity 1.0), the formula is shaft power (bhp) = (Q x H x SG) / (3960 x efficiency), with the result also converted to kW. The lower the pump's efficiency, the more shaft power is needed to deliver the same flow and head.
When selecting an actual motor, it's standard practice to add a 10-20% service factor above the calculated shaft power. If you're pumping a fluid with a different specific gravity or viscosity than water, or need a precise design, treat this result as a starting point and confirm with a mechanical engineer.
Frequently Asked Questions
Water horsepower (whp) is the theoretical power needed to move the fluid alone. Shaft/brake horsepower (bhp) adds the pump's mechanical losses (efficiency), representing what the motor must actually deliver. Shaft power = water power / efficiency.
No. It's standard practice to add a service factor margin — typically 10-20% above shaft power — when selecting motor rating, to cover startup load variation and drivetrain losses like belt transmission.