The computation of nonreciprocal mode conversion characteristics of magnetooptic waveguides is thoroughly described by means of full and semi-vectorial finite element approaches. In particular, the mode-conversion maximum isolation ratio of a magnetooptic rib guide is evaluated and compared with updated experimental and numerical results available in the literature. The latter being based on a semi-vectorial finite element formulation. The improvement achieved by using a full vectorial formulation is clearly demonstrated. In addition a more efficient way to compute the semi-vectorial finite element approach currently used in the literature, is given in detail.