The relaxation volume ( rel), here determined per extra-atom or vacant site, of common crystalline defects in bcc iron (Fe) was calculated from molecular dynamics simulation (MD) cells containing defects of varying size and/or density. To this end, we used both real and reciprocal space data: for the former, the change in the MD cell volume was calculated, while for the latter, we computed X-ray diffraction reciprocal space maps to evaluate the change in the lattice parameter. We show that <110> dumbbell self-interstitial atoms have the largest  rel , ~1.5 atomic volume (~1.5 0). C15 clusters of size 12 and 48 atoms show  rel of ~0.91 0 and ~0.98 0, respectively, and similar values are found for ½<111> and <100> interstitial dislocation loops, with  rel ~0.905 0 and  rel ~0.873 0, respectively. Single vacancies are characterized by a negative  rel , ~-0.11 0. For cavities,  rel rapidly increases to approach zero as the clusters grow. Using these values, we managed to predict (with an accuracy better than 2 %) the lattice strain in MD cells containing several types of defects, which indicates that the relaxation volumes can be summed up to estimate the microscopic (i.e., lattice) volume change.