Inclusion size distribution and shape are typically determined from the analysis of a polished cross-section of steel specimen. However, measured inclusion characteristics such as the 2D size distribution and shape can differ from the actual 3D characteristics that influence the mechanical properties of steel. In this study, MnO·SiO2 inclusions that were liquid at steelmaking temperatures have been extracted from a Mn-Si killed steel using an electrochemical dissolution method. Automated SEM/EDX analysis was used to perform a direct comparison of the inclusions observed in the 2D polished section and in 3D by analyzing extracted inclusions collected on a filter. A spherical inclusion shape was assumed and used to perform the statistical conversion of the measured 2D particles to a virtual 3D particle size distribution and the predicted distribution was compared to the measured 3D distribution. The causes of the departure of the real 3D inclusion statistics from the converted 2D data are discussed.