Head is the term used to describe the energy imparted to the liquid. The units of head are foot-pounds (ft-lb) of force per pound of mass. Note that the important velocity is the tangential velocity at the tip of the impeller. This velocity is proportional to the diameter of the impeller and the rotational speed. Therefore, the equation for head can be written in terms of pump characteristics as follows: As shown in Figure 3, the precise units of head are ft-lb (force) per lb (mass). However, it is conventional practice to cancel the lb units and to speak of head in terms of feet. Note that the pump vendor designs the impeller to produce the head required at the design point.

The pressure differential produced by a pump is equivalent to a column of the pumped liquid, where the height of the column is equal to the head produced by the pump. For a given flow and speed, a given centrifugal pump produces a constant head with any fluid, assuming no wear and fouling. The S.G. is the specific gravity at flowing conditions.