The student-faculty ratio is of great significance to policy makers and media as a popular measure of education and teaching quality. Due to its simplicity and the availability of data, it is often used in higher education policy for allocating resources and for ranking universities. This is especially so in some European countries which do not have selective admission policies and where universities have to cope with huge numbers of students. However, there is no definition and no empirically validated data for an appropriate student-faculty ratio. To close this gap, we constructed a model with parameters relevant for high quality teaching and education and validated them empirically by conducting a survey among university professors in business administration. The results clearly illustrate that student-faculty ratios are discipline specific and depend whether the university is research or teaching oriented.