Inclusion size distributions are typically determined by the analysis of a polished cross-section of a steel specimen. However, the measured 2D distribution differs from the actual 3D size distribution. In this study, Al2O3 and MnO·SiO2 inclusions have been extracted using an electrochemical dissolution method to reveal the actual size and shape of the 3D inclusion population, and a direct comparison of the 2D and 3D size distributions has been performed. Statistical methods for converting 2D to 3D particle size distribution have also been examined and compared to the measured 3D distribution.