Open porosity is one of the most important parameters in the characterisation of a porous ceramic. Two methods based on the Archimedes' Principle, ISO 2738 and EN623-2 are currently used as standards for the measurement of open porosity. The method in ISO 2738 was found to underestimate the open-pore content, especially in fine-pored ceramics. In contrast, the EN623-2 method was found to more accurately measure the true open-pore content of the materials. A mathematical model to describe the phenomenon is proposed.