Impeller effectiveness has often been evaluated via either mixing time, theta(m), or flow number, Fl; and a direct connection between the two has also been assumed in some cases. Here, these concepts are considered in the light of recent theoretical and experimental work. It is shown that an equation for mixing time recently published by BHR Group can be related to a basic turbulence model. It is also shown that this equation is superior to those based on a theory linking mixing time to the flow generated by the impeller. The new equation and theory implies that all impeller types of equal impeller-to-tank diameter ratio are equally energy efficient in achieving overall homogenisation. On the other hand, impeller efficiency based on the flow generated, as measured by laser Doppler anemometry at equal measured power, suggests a significant difference between impeller types.