This paper presents a new reactive power-based performance index of planar mechanisms. This index quantifies how much of the power introduced to a mechanism is transformed into motion. Generally, the mechanical advantage is used as an index to quantify the force or the velocity ratio between the input and the output link of a mechanism. However, this index is sensitive to the dimensions of the inlet and outlet of the mechanism so it cannot be used for the comparison of different types of mechanisms. To overcome the drawback of the physical inconsistency, the concept of reactive power is proposed to quantify the losses that occur in a mechanism and, based on this concept, the reactive power-based index is defined. The proposed approach is applied to various mechanisms and it is compared with the mechanical advantage to observe that the proposed index provides the same information as the mechanical advantage, but being dimensionally consistent for the analyzed cases (dimensionless in all cases), it can be used to the analysis, comparison and optimal design of different planar mechanisms.