Dead Space Calculator — Pulmonary VD/VT Ratio
Calculate the dead space volume (the portion of each breath that does not reach the alveoli for gas exchange), the VD/VT ratio, and the effective alveolar ventilation — using either a standard anatomical estimate or the Bohr equation with CO₂ partial pressures.
mL
breaths/min
Dead space method
Volume of each breath that does not participate in gas exchange
- 1
VD/VT ratio
150 ÷ 500 = 0.3Using standard anatomical dead space of 150 mL for a 70 kg adult. - 2
Dead space volume (VD)
150
How does this calculator work?
Dead space is the portion of each breath not reaching gas-exchange alveoli (~150 mL anatomically, or derived from the Bohr equation using PaCO₂ and PēCO₂). Alveolar ventilation = (VT − VD) × RR. Normal VD/VT ratio is 0.25–0.35; above 0.60 signals impaired gas exchange.
Formula
How this is calculated
Every breath fills two compartments: the alveolar volume (where O₂ and CO₂ actually exchange with blood) and the dead space (airways and any non-perfused alveoli that carry gas in and out without exchanging it). The anatomical dead space — the conducting airways from mouth to terminal bronchioles — is roughly 150 mL in a healthy 70 kg adult, often approximated as 1 mL per pound of body weight.
Physiological dead space is larger because it also includes alveoli that are ventilated but not perfused (V/Q mismatch). The Bohr equation derives it indirectly from CO₂: VD/VT = (PaCO₂ − PēCO₂) / PaCO₂, where PaCO₂ is the arterial partial pressure of CO₂ and PēCO₂ is the mixed expired CO₂ averaged over a whole breath. A normal VD/VT ratio is 0.25–0.35 at rest; values above 0.60 indicate severely impaired gas exchange.
Alveolar ventilation (V̇A = (VT − VD) × RR) is the clinically important figure — it is the volume per minute that actually reaches the alveoli and drives CO₂ elimination. Increased dead space (pulmonary embolism, ARDS, emphysema) reduces alveolar ventilation for a given tidal volume and rate, requiring higher minute ventilation to maintain normocapnia.
Frequently asked questions
At rest in healthy adults the ratio is 0.25–0.35 (25–35% of each breath is wasted). Mechanical ventilation patients often have higher ratios (0.40–0.55). Values above 0.60 are associated with poor gas exchange and high weaning failure risk.
Anatomical dead space is the volume of conducting airways (mouth, trachea, bronchi) that carry gas but do not exchange it — approximately 150 mL. Physiological dead space adds any alveoli that are ventilated but not perfused (V/Q mismatch). In healthy lungs they are nearly equal; in lung disease physiological dead space can be much larger.
A high VD/VT ratio means more of each breath is wasted, so minute ventilation must increase to maintain target PaCO₂. Clinicians compensate by increasing tidal volume, respiratory rate, or both — but large tidal volumes risk barotrauma, so the Bohr equation guides this balance.
Also known as
TG we-Calculate Editorial Team. (2026). Dead Space Calculator — Pulmonary VD/VT Ratio [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/dead-space-calculator
TG we-Calculate Editorial Team. "Dead Space Calculator — Pulmonary VD/VT Ratio." TG we-Calculate. 2026. https://we-calculate.com/calculator/dead-space-calculator.
TG we-Calculate Editorial Team, "Dead Space Calculator — Pulmonary VD/VT Ratio," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/dead-space-calculator
@misc{wecalculate_dead_space_calculator, title = {Dead Space Calculator — Pulmonary VD/VT Ratio}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/dead-space-calculator}}, year = {2026}, note = {TG we-Calculate} }
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