Lagomorphs are widespread around the world, but little is known about the biology and ecology of their fossil ancestors. In this case, knowing the body mass of these extinct species is of principal interest because it is correlated with physiological, morphological and life history attributes. Moreover, insular fossil rabbits, hares and pikas, which became spectacular giants with huge weights and dramatic shifts in their life histories, encourage curiosity in the research world. Our principal aim is to create allometric models between skeletal parameters and body weights with extant species of the order Lagomorpha (both ochotonids and leporids). These regressions can then be applied to the fossil register to estimate the body mass of the extinct lagomorphs. The models are satisfactory in all cases, although weaker relationships were obtained when we analyzed dental parameters. Multiple models have slightly better results than bivariate ones, but their use is limited to complete bones or skeletons. These body mass estimation models were tested in three different fossil lagomorphs: Prolagus apricenicus, Prolagus cf. calpensis and Nuralagus rex. In all three cases, the results from dental variables were discarded due to the fact that these species may not follow the allometric relationship between teeth and body mass of standard lagomorphs. Other variables, such as the proximal anteroposterior diameter of the humerus in N. rex, were also removed for their implications in fossorial lifestyle. We ultimately estimated a weight of around 600 g for P. apricenicus, 300 g for P. cf. calpensis and 8000 g for N. rex. Differences in extrinsic mortality explain the important differences in body masses between the two Prolagus species. The results of N. rex cannot be compared with the giant Prolagus due to phylogenetic differences.