Pump Horsepower Calculator: engineer's guide
Pump brake horsepower is the shaft power to deliver the duty; motor size is BHP divided by motor efficiency plus a service factor.
Open the Pump Horsepower calculator →
What this calculator does
The calculator returns hydraulic HP, brake HP at the operating efficiency, and recommended motor kW rating.
Formulas used
- Water horsepower:
WHP = GPM × H (ft) / 3960 - Brake horsepower:
BHP = WHP / ηpump - Motor kW:
kW = BHP × 0.746 / ηmotor
Variables
- ηpump = 0.60 to 0.85 typical
- ηmotor = 0.85 to 0.95 typical
Worked example
Scenario: 400 GPM at 100 ft TDH, pump efficiency 0.72, motor efficiency 0.92.
- WHP = 400 × 100 / 3960 ≈ 10.1 HP
- BHP = 10.1 / 0.72 ≈ 14.0 HP
- Motor kW = 14 × 0.746 / 0.92 ≈ 11.4 kW → select 15 kW motor
Common mistakes to avoid
- Sizing motor at BHP exactly — overload during start
- Using nameplate efficiency instead of operating-point efficiency
- Ignoring service factor for continuous duty
Frequently asked questions
Standard motor kW?
0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200 kW.
Motor efficiency class?
IE3 minimum for new installations per most jurisdictions.
Standards and references
ANSI/HI 1.3, IEC 60034-30
Related tools on this site
Open the calculator that pairs with this guide: Pump Horsepower Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.