Operator calculator
Brake and Motor Horsepower calculator
Account for pump and motor efficiency to find the motor horsepower required.
Calculator
Motor horsepower
33.67 hp
- 1. Water hp = 1,000 × 90 ÷ 3,960 = 22.73 hp
- 2. Brake hp = 22.73 ÷ 0.75 = 30.3 hp
- 3. Motor hp = 30.3 ÷ 0.9 = 33.67 hp
For instructional use. Results assume the standard conversion factors shown on this page. Actual field calculations may need chemical strength, density, temperature, and other adjustments, so check your plant’s procedures.
Formula and inputs
motor hp = [flow (gpm) × head (ft) ÷ 3,960] ÷ pump efficiency ÷ motor efficiency
| Input | Unit |
|---|---|
| Flow | gpm |
| Total head | ft |
| Pump efficiency | % |
| Motor efficiency | % |
| Result: Motor horsepower | hp |
Conversion factors used: 3,960 for gpm and feet of head.
When to use it
A pump needs more power at the shaft than the water receives, and a motor needs more power than the shaft uses. Each step loses some to inefficiency.
Divide water horsepower by pump efficiency to get brake horsepower, then divide again by motor efficiency for motor horsepower.
Applies to: Water Treatment, Water Distribution, Wastewater Treatment, Collection Systems.
Exam tip: Efficiency always makes the required horsepower go up. If your answer got smaller, you multiplied.
Study guides
Build the math skill behind the calculator
Water Operator Math Made Simple walks through the formulas step by step.
Water Operator Math Made Simple
A Step-by-Step Guide to the Math Every Water Operator Needs to Know