In many industrial areas, downsizing the pumping system is a decisive aim of the designers. The reasons could be multiple means; in a single-stage pump, increasing the power density of the pump means actually reducing the production costs. The main goal of this study was the comparison in terms of power density of a conventionally designed single-stage pump with a novel design concept based on the counter-rotating (CR) principle. In order to simplify the experimental investigations for the present study, the volute geometry was fixed instead of reducing the pump outflow diameters for a fixed design point. The energy concentration was then increased by raising the developed hydraulic power within the same envelope. The design of the impellers was carried out with an in-house design tool, based on inverse design method. Numerical results highlight the advantageousness of the new layout, in terms of power concentration, compared to the conventional impeller. Numerical predictions are also in significant agreement with the experimental investigation results, obtained in a specifically developed CR motors test rig. The experimental optimization of the rotational speed ratio of the CR impellers has shown the possibility to further increase the head in off-design condition and thereby the pump power density.