The volume of air remaining in the lungs after normal breathing is called functional residual capacity (FRC). It combines expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximum volume of air that can be exhaled after a normal expiration and it is about 1000 mL to 1500 mL. RV, on the other hand, refers to the volume of air remaining in the lungs after maximum expiration and is about 1100 mL to 1500 mL.
Thus, FRC = ERV + RV
1500 + 1500 = 3000 mL
Hence, functional residual capacity of the human lungs is about 2500 - 3000 mL.