International comparisons of pension fund charges are important in identifying cost- effective structures for funded retirement systems. Heterogeneity in the type of base used to set management fee rates across jurisdictions constitutes a barrier for this type of studies. To overcome this, a closed formula is derived to estimate the contributions-based rate equivalent to a given assets-based rate. Besides enabling international comparisons, the formula allows to calculate the net returns of funds with contribution-based fees. The results of an empirical application in a set of Latin American countries suggest that the conclusions about fee drivers in retirement systems can vary significantly depending on the rate conversion method used.