Oxygenation Index = Arterial Oxygen Pressure PaO₂ / Inspired Oxygen Concentration FiO₂

In clinical practice, PaO₂/FiO₂ is most commonly used. PaO₂ is measured in mmHg, and FiO₂ is the fraction of inspired oxygen. For example, if a patient has a PaO₂ of 80 mmHg while breathing 40% oxygen, then PaO₂/FiO₂ = 80 / 0.4 = 200 mmHg.

The normal value for PaO₂/FiO₂ is 400-500 mmHg. A value ≤300 mmHg is a criterion for diagnosing acute respiratory distress syndrome.