Field studies of use of torpor by heterothermic endotherms suffer from the lack of a standardized threshold differentiating torpid body temperatures (T(b)) from normothermic T(b)'s. This threshold can be more readily observed if metabolic rate (MR) is measured in the laboratory. I digitized figures from the literature that depicted simultaneous traces of MR and T(b) from 32 respirometry runs for 14 mammal species. For each graph, I quantified the T(b) measured when MR first began to drop at the onset of torpor (T(b-onset)). I used a general linear model to quantify the effect of ambient temperature (T(a)) and body mass (BM) on T(b-onset). For species lighter than 70 g, the model was highly significant and was described by the equation Tb-onset=(0.055+/-0.014)BM+(0.071+/-0.031)Ta+(31.823+/-0.740). To be conservative, I recommend use of these model parameters minus 1 standard error, which modifies the equation to Tb-onset-1 SE=(0.041)BM+(0.040)Ta+31.083. This approach provides a standardized threshold for differentiating torpor from normothermia that is based on use of energy, the actual currency of interest for studies of torpor in the wild. Few laboratory studies have presented the time-course data required to quantify T(b-onset), so more data are needed to validate this relationship.