While the density of materials is important for their mechanical and service properties, it is striking that so little is known in the case of silicon cast irons, which are essentially Fe-C-Si alloys with graphite precipitates in an iron-rich matrix. In this work, a review of literature data was used to establish an expression for the lattice parameter of ferrite based on that of pure iron, to which were added the individual effects of carbon, silicon, manganese, phosphorus and copper. The expression thus obtained is valid for a homogeneous material, and a correction is proposed to take account of the presence of graphite in Fe-C-Si ferritic alloys. Next, the ferrite lattice parameter of a series of 12 alloys containing carbon between 1.0 and 4.1 wt.%, silicon between 0.5 and 4.3 wt.%, up to 0.7 wt.% Mn and up to 0.9 wt.% Cu, was measured by XRD from room temperature to a temperature of 1150°C. These recordings showed that the mirror polishing used to prepare the samples resulted in surface alteration and a significant overestimation of the ferrite lattice parameter for temperatures below 500°C. We therefore carried out high-energy XRD at room temperature, which was free of any surface effects and validated our analysis. Valid measurements at room temperature and XRD values obtained at temperatures above 500°C were then successfully compared with the predictions given by the proposed expression for the lattice parameter of ferrite as a function of temperature and alloying elements. This work provides the necessary information to calculate the theoretical density of ferritic cast irons.