Amongst the possible defects that can appear during casting, voids due to solidification shrinkage is certainly the most usual and important in the case of cast irons. For any predicting work, the knowledge of the density of the phases of interest, namely liquid, austenite and graphite, is a prerequisite. The density of liquid and austenite in silicon cast irons are here assessed as function of composition and temperature based on literature review. Estimate of theoretical expansion upon solidification are then derived for lamellar and spheroidal graphite cast irons and compared to reported experimental values obtained in nearequilibrium conditions.