In short: pump efficiency is the hydraulic power delivered to the water divided by the shaft power put into the pump, as a percentage. For the default example above — 100 GPM at 100 ft of head on a 5 hp pump — that works out to about 50.5%. Centrifugal pumps typically run 40-85% efficient depending on size and how close the operating point is to the pump's best-efficiency point.
This is a planning-aid calculation based on the shaft power you enter; it is not a substitute for a certified pump performance test. For procurement or warranty decisions, confirm efficiency against the manufacturer's certified curve.
How to calculate pump efficiency
A pump never converts all the power driving its shaft into useful work moving water — friction, turbulence and internal leakage waste some of it as heat. Efficiency is simply the ratio of what comes out (hydraulic power) to what goes in (shaft power):
This calculator converts your flow and head into SI units, computes hydraulic power directly from the water's density, and compares it to the shaft power you entered. The same single calculation drives every figure shown, so the hydraulic power, power loss and efficiency always agree with each other.
Worked example. Take 100 US GPM at 100 ft of head on fresh water, with 5 hp measured at the shaft. 100 GPM is 0.00630902 m³/s and 100 ft is 30.48 m. Hydraulic power = 998.2 kg/m³ × 9.80665 m/s² × 0.00630902 m³/s × 30.48 m = 1,882.4 W, or 2.52 hp. Shaft power is 5 hp (3,728.5 W). Efficiency = 1,882.4 ÷ 3,728.5 = 50.5%.
Shaft power, not motor input. This formula uses power delivered to the pump shaft. If your only figure is electrical power drawn by the motor, that includes motor losses too — see Pump Energy Cost Calculator for working with metered electrical input instead.
Why flow and head change the answer
Every pump has a best-efficiency point (BEP) — the flow and head combination where internal losses are lowest. Move away from the BEP in either direction and efficiency drops: too little flow for the head causes recirculation inside the casing, while too much flow increases friction losses faster than useful output grows. That is why the same pump can read 70% efficient at one operating point and 45% at another.
If you are comparing efficiency across different pump speeds or impeller sizes rather than different operating points on the same curve, the Pump Affinity Laws Calculator is the right tool — it relates how flow, head and power scale with speed.
Typical pump efficiency by size and type
Approximate, illustrative ranges for rotodynamic (centrifugal) pumps. Always check the manufacturer's certified performance curve for a specific model — efficiency varies with operating point, not just size.
| Pump type / size | Typical efficiency |
|---|---|
| Small household pumps (under 1 hp) | 35-50% |
| Small centrifugal pumps (1-5 hp) | 45-65% |
| Mid-size industrial centrifugal (10-75 hp) | 65-80% |
| Large multistage / municipal pumps (100+ hp) | 80-90% |
| Positive-displacement pumps, at rated point | 85-95% |
Worked examples
Small booster pump
50.5%
100 GPM at 100 ft of head, 5 hp shaft input, fresh water: 2.52 hp hydraulic power against 5.00 hp shaft power in.
Submersible well pump
63.1%
50 GPM at 150 ft of head, 3 hp shaft input: 1.89 hp hydraulic power against 3.00 hp shaft power in — closer to a typical BEP for this size.
Municipal booster pump
67.3%
500 GPM at 80 ft of head, 15 hp shaft input: 10.10 hp hydraulic power against 15.00 hp shaft power in, typical of a well-matched mid-size unit.
What else affects the result
Motor efficiency. This calculator reports pump efficiency alone, using shaft power. If you power the pump through an electric motor, the motor's own losses sit on top — the "wire-to-water" efficiency is pump efficiency multiplied by motor efficiency, and is always lower than either alone.
Cavitation and NPSH. Insufficient net positive suction head causes cavitation, which drops efficiency sharply and can damage the impeller over time. Check available NPSH with the NPSH Calculator if efficiency reads unexpectedly low.
Wear and clearances. Worn wear rings, impeller erosion and increased internal clearances let more water recirculate inside the pump instead of being pumped out, lowering efficiency as a pump ages.
Fluid viscosity. This calculator assumes water. Thicker fluids increase friction losses inside a centrifugal pump and reduce efficiency below the water-based rating on the pump curve.
Frequently asked questions
What is a good pump efficiency?
It depends heavily on pump size and type. Small residential pumps (under 5 hp) often run 40-65% efficient, mid-size industrial centrifugal pumps 65-80%, and large multistage or municipal pumps 80-90%. A pump running well below the typical range for its size and type may be operating far from its best-efficiency point, worn, or undersized for the job.
How is pump efficiency calculated?
Overall pump efficiency is hydraulic power divided by shaft input power, expressed as a percentage: efficiency = (density × gravity × flow × head) ÷ shaft power. Hydraulic power is the useful power actually delivered to the water; shaft power is the mechanical power driving the pump.
Why is my measured pump efficiency low?
The most common causes are operating far from the pump's best-efficiency point (too much or too little flow for the head), worn impellers or wear rings, insufficient NPSH causing cavitation, or a fluid more viscous than water. Comparing your flow and head against the manufacturer's performance curve usually shows which is happening.
Does water temperature affect pump efficiency?
Only slightly for cold or hot water. Density changes a few percent between 4°C and 60°C, which shifts hydraulic power by a similar small amount. The bigger risk with hot water is a higher chance of cavitation, since vapor pressure rises with temperature and reduces the available NPSH margin.
Is this pump efficiency or motor efficiency?
This calculator reports pump (hydraulic) efficiency, using the shaft power actually delivered to the pump. If you only know the electrical power drawn by the motor, multiply that by the motor's efficiency first to estimate shaft power, or the result will understate true pump efficiency.
Sources & references
- Hydraulic Institute. ANSI/HI 9.6.6, Rotodynamic Pumps for Pump Efficiency Testing.
- Karassik, I. J., Messina, J. P., Cooper, P., & Heald, C. C. (Eds.). Pump Handbook (4th ed.). McGraw-Hill.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)