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). It is calculated by the formula -: FRC = ERV + RV.

Where

ERV is the maximum volume of air that can be exhaled after a normal expiration. It is about 1000 mL to 1100 mL.

RV is the volume of air remaining in the lungs after maximum expiration. It is about 1100 mL to 1200 mL.

Thus 

FRC = ERV + RV
FRC = 1100 + 1200
FRC =  2300 mL

Functional residual capacity of the human lungs is about 2.1 - 2.3 L.