What is dead space
Physiological dead space is the portion of each breath that does not participate in gas exchange — air that fills the airways and any poorly perfused alveoli without contributing to oxygen and carbon dioxide exchange with the blood.
The Bohr equation
This calculator uses the Bohr equation: Vd/Vt = (PaCO2 − PeCO2) ÷ PaCO2, where PaCO2 is arterial carbon dioxide and PeCO2 is mixed expired carbon dioxide. The resulting fraction, multiplied by tidal volume, gives the estimated physiological dead space volume. A normal Vd/Vt ratio is typically around 0.2 to 0.35 in healthy adults.
This is a respiratory physiology reference tool for general educational purposes and does not replace measurement or interpretation by a qualified healthcare professional.
Last reviewed August 2026