Although the energy codes and standards for buildings have been considerably improved in the last decades, there are consistent reports on building damage caused by mold growth. Numerical models for predicting mold growth can be useful in assessing the risk of mold growth in new construction and retrofit applications. Two well-known models exist and are widely used: the Viitanen (VTT) model and the biohygrothermal model. While the VTT model is an empirical model, the biohygrothermal method is based on a theoretical model and considers transient ambient conditions. The result is mold growth in millimeters, which is, however, not intelligible to all. The Scandinavian countries in particular established a very clear six-step evaluation model, the so-called mold index. By combining the results of the biohygrothermal model and the mold index of the VTT model, it is possible to use a clear and acknowledged rating measure. A conversion function was developed allowing the transformation of the calculated growth, in millimeters, into the mold index with a high level of correspondence.