In an normal adult, the anatomical dead space is approximately:

Approximately 30% of each breath does not paicipate in alveolar gas exchange and is thus called dead space ventilation (VDS). Dead space ventilation is made up of anatomic dead space and alveolar dead space. Anatomic dead space is the volume of air that fills the conducting airways (trachea, bronchi, and bronchioles) and is estimated at approximately 2.2 cc/kg. The combination of alveolar and anatomic dead space is called the physiologic dead space. The normal ratio between dead (VDS) space ventilation and tidal volume (VT) is 0.3. Ref: Slesinger T.L. (2011). Chapter 20. Blood Gases. In R.K. Cydulka, G.D. Meckler (Eds), Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 7e.