Pump Motor Horsepower Calculator for Water

Pump Horsepower Formula:

\[ HP = \frac{Q \times H \times SG}{3960 \times \eta} \]

Flow Rate (Q):

GPM

Head (H):

feet

Specific Gravity (SG):

(1.0 for water)

Pump Efficiency (η):

decimal (0-1)

Required Motor Horsepower (HP):

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1. What is Pump Horsepower Calculation?

The pump horsepower calculation determines the power required to move a fluid at a specified flow rate against a given head (pressure). It's essential for selecting the right pump motor for water systems, irrigation, and industrial applications.

2. How Does the Calculator Work?

The calculator uses the standard pump horsepower formula:

\[ HP = \frac{Q \times H \times SG}{3960 \times \eta} \]

Where:

\( Q \) — Flow rate in gallons per minute (GPM)
\( H \) — Total dynamic head in feet
\( SG \) — Specific gravity of the fluid (1.0 for water)
\( \eta \) — Pump efficiency as a decimal (0-1)
3960 — Conversion factor (foot-pounds per minute per horsepower)

Explanation: The formula calculates the theoretical power required and then divides by pump efficiency to get actual motor horsepower needed.

3. Importance of Proper Horsepower Calculation

Details: Correct horsepower calculation ensures the pump motor can handle the required workload without being oversized (wasting energy) or undersized (causing failure). It affects energy costs, system reliability, and equipment lifespan.

4. Using the Calculator

Tips:

For water systems, specific gravity is typically 1.0
Pump efficiency ranges from 0.5-0.95 (50-95%) depending on pump type and condition
Always round up to the nearest standard motor size
Include safety factors in final motor selection

5. Frequently Asked Questions (FAQ)

Q1: What's a typical efficiency value for pumps?
A: Centrifugal pumps typically range 70-85%, while positive displacement pumps can reach 90-95%. Always check manufacturer specifications.

Q2: How do I account for motor efficiency?
A: This calculation gives brake horsepower. To get electrical input power, divide by motor efficiency (typically 0.85-0.95 for electric motors).

Q3: What's the difference between WHP and BHP?
A: Water Horsepower (WHP) is theoretical power (η=1), Brake Horsepower (BHP) includes pump efficiency (this calculator's result).

Q4: Should I add a safety factor?
A: Yes, most applications add 10-25% to the calculated HP to account for system variations and future needs.

Q5: How does viscosity affect the calculation?
A: High viscosity fluids require more power. Use viscosity correction factors or consult pump performance curves for non-water applications.