A pump converts the energy used to turn its shaft into water energy. The efficiency with which it does so determines what it costs to move the liquid. It takes the same amount of energy to lift one gallon of water two feet as it does to lift two gallons one foot. The formula for water is: Horsepower = (Gallons per minute) x (Total dynamic head in feet) 3960 x pump efficiency This is derived from the definition of horsepower and is always true. For liquids other than water, the specific gravity must be used but that is for another web page. 3960 is a conversion factor to make the units come out right. The word dynamic means the total head is figured when the liquid is in motion and so friction losses must be included.