Formulas for the effective volumes and effective surfaces for rectangular bars loaded in flexure are reviewed. The ratio of strengths from any two configurations, such as three-point flexural to four-point, is independent of whether the flaws are volume- or surface-distributed, if the cross-sectional sizes are the same. Discrimination between volume- or surface-distributed flaw types, based on the strength ratios, is not possible in such cases. On the other hand, conversion of strengths from one configuration to another is very easy if testing is performed in accordance with one of the current world standard test methods.