Abstract Bulk metallic glasses are out of equilibrium materials and consequently they tend to crystallize when heated in the supercooled liquid region. Crystallization has been shown to change drastically the properties of the BMG. However, in order to quantify these changes due to crystallization, it is first necessary to get detailed information about the structure evolution and especially the size and volume fraction of the crystallites. Partial crystallization of the Vitreloy 1 metallic glass is carried out in the supercooled liquid region. The crystallization is quantified by DSC and XRD measurements and the obtained values are compared. An important difference is observed and the maximum volume fraction is compared to the value predicted by appropriate numerical simulations. It is shown that XRD measurements seem to be appropriate to calculate the crystallite volume fraction while care must be taken if using DSC measurements.