The volume of air remaining in the lungs after a normal expiration is known as functional residual capacity (FRC). It includes expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximum volume of air that can be exhaled after a normal expiration. It is about 1000 mL to 1500 mL. RV is the volume of air remaining in the lungs after maximum expiration. It is about 1100 mL to 1500 mL.

Thus, FRC = ERV + RV

≅ 1500 + 1500

≅ 3000 mL

Functional residual capacity of the human lungs is about 2500 – 3000 mL.