Alveolar–Arterial Oxygen Gradient Calculator
Estimate alveolar oxygen tension and the A–a oxygen gradient at sea level
Version 1.0.0
Alveolar oxygen tension (PAO2)
— mmHg
A–a oxygen gradient
— mmHg
Assumes sea-level barometric pressure (760 mmHg), water vapour pressure of 47 mmHg and a respiratory quotient of 0.8. Results may not apply at altitude.
No clinical values are intentionally saved by this control.
Equations
Alveolar gas equation (sea level)
PAO2 = FiO2 × (PB - PH2O) - PaCO2 / RQ = (713 × FiO2) - (1.25 × PaCO2)PAO2 is alveolar oxygen partial pressure (mmHg); FiO2 is inspired oxygen fraction; PB is fixed at 760 mmHg; PH2O is fixed at 47 mmHg; PaCO2 is arterial carbon dioxide partial pressure (mmHg); and RQ is fixed at 0.8. The displayed PAO2 is rounded to the nearest whole mmHg before calculating the gradient, preserving the legacy calculation sequence.
Alveolar–arterial oxygen gradient
A–a gradient = rounded PAO2 - PaO2PaO2 is arterial oxygen partial pressure (mmHg). The final gradient is rounded to the nearest whole mmHg.
References
- Hendrix JM, Burns B. Alveolar Gas Equation. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Updated 2024 Sep 11. PMID: 29489223