The objective of the present study was to explore the correlation between resting metabolic rate (RMR), body composition and physical activity in active and inactive men and women aged 20-30 years. In total, 13 active and 10 inactive women and 8 active and 5 inactive men were enrolled in this study. RMR was measured using an indirect respiratory calorimeter and body composition was obtained using a BodPod, anthropometric measurements (measurements of waist, weight, skin fold thickness etc.) and bioelectrical impedance analysis. From the BodPod were information obtained about fat-free mass (FFM) and fat mass (FM) and from the bioelectrical impedance analysis were data obtained about the total body water (TBW). By combining the data from these methods a three-compartment model could be produced. This gives a better value of the body composition. By using anthropometric measurements the muscle mass (MM) could be calculated. FFM measurements correlate best with RMR for both active men and women and for inactive women. For the inactive men MM correlates best with RMR. In conclusion, comparing the groups as a whole it is seen that the FFM correlates best with RMR for both women and men. Since the muscles are the metabolically active part of FFM it is concluded that muscles affects the RMR value and muscles can in turn be influenced by training.