Studies of ecological energetics in chelonians are rare. Here, we report the first measurements of daily energy expenditure (DEE) and water influx rates (WIRs) in free-ranging adult Gopher Tortoises (Gopherus polyphemus). We used the doubly labeled water (DLW) method to measure DEE in six adult tortoises during the non-breeding season in south-central Mississippi, USA. Tortoise DEE ranged from 76.7–187.5 kJ/day and WIR ranged from 30.6–93.1 ml H₂O/day. Daily energy expenditure did not differ between the sexes, but DEE was positively related to body mass. Water influx rates varied with the interaction of sex and body mass. We used a log/log regression model to assess the allometric relationship between DEE and body mass for Gopher Tortoises, Desert Tortoises (Gopherus agassizii), and Box Turtles (Terrapene carolina), the only chelonians for which DEE has been measured. The slope of this allometric model (0.626) was less than that previously calculated for herbivorous reptiles (0.813), suggesting that chelonians may expend energy at a slower rate per unit of body mass compared to other herbivorous reptiles. We used retrospective power analyses and data from the DLW isotope analyses to develop guidelines for sample sizes and duration of measurement intervals, respectively, for larger-scale energetic studies in this species.