In the horsepower equation HP = (Flow × Head × Weight) / 3960 × Pump Efficiency, what constant appears in the denominator?

• A. 1000
• B. 7920
• C. 3960
• D. 2040

Answer: (C)

The concept here is converting hydraulic power into horsepower using a standard unit conversion that ties flow, head, and liquid weight to horsepower. The constant in the denominator comes from turning lb‑ft per minute into horsepower. One horsepower equals 33,000 ft-lb per minute. Since the formula uses weight per gallon (lb/gal) with flow in gallons per minute, multiplying flow × head × weight gives you lb-ft per minute, and you divide by 33,000 to get horsepower. For water, weight is about 8.34 lb/gal, so 33,000 divided by 8.34 is roughly 3960. That’s why the denominator is 3960 in this expression. If you pumped a liquid with a different weight per gallon, the denominator would adjust accordingly (33,000 divided by that weight).