Pump Water Horsepower

Water horsepower is the honest output of a pump: the weight of liquid lifted per second times the height it is lifted. In US customary units that reduces to gpm × ft × SG ÷ 3960, where the 3960 is just 33 000 ft·lbf/min per horsepower divided by 8.33 lb per gallon of water. Pump 500 gpm to 100 ft of head on water and you are delivering 500 × 100 × 1.0 / 3960 ≈ 12.6 hp to the liquid — this calculator works in exact SI (ρgQH with g = 9.80665) and returns 12.64 hp, the small difference being the rounding baked into 3960.

The specific-gravity term is where people come unstuck. Head in feet is independent of density, but power is not: the same pump moving 1.2 SG glycol brine to the same 100 ft of head needs 20% more horsepower even though the gauge pressure and the curve point are unchanged. Size the motor for the coldest, densest fluid the system will ever see, including a winter glycol charge that maintenance may add years later.