The primary purpose of this study was to assess the reliability of 3 methods estimating energy expenditure (EE) during and in response to resistance exercise. Ten males (aged 29.4 ± 10.2 years) with ≥2 months resistance training (RT) experience performed 3 training sessions incorporating the bench press and back-squat; sessions were separated by 48 to 72 h. Total energy expenditure (TEE) was estimated using a Suunto T6D Heart Rate Monitor and 2 methods (named “Scott” and “Magosso”) that used oxygen uptake and blood lactate measurements to determine aerobic and anaerobic energy expenditure (AnEE). For TEE, relative reliability for both the Scott and Magosso methods remained “nearly perfect” across all testing days for the bench press and back-squat; with interclass correlations (ICC) > 0.93 and percentage of the typical error measurement (TEM%) below 5.8%. The heart rate method showed moderate variability between testing days for both exercises; ICCs ranged between 0.66–0.92 with TEM% between 18%–37% during the bench press and 11%–17% during the back-squat. The estimation of AnEE showed that the Scott and Magosso methods had “strong” to “very strong” relative reliability for both exercises; however, a low absolute reliability was observed. Mean EE was significantly higher in the Scott and Magosso methods during the bench press >912 kJ and back-squat >1170 kJ, with the heart rate method estimating 358 kJ and 416 kJ. The Scott and Magosso methods showed a high degree of reliability between testing days when measuring EE. Heart rate methods may significantly underestimate EE during and in response to RT.