Acute Lung Injury (ALI) is a precursor to Acute Respiratory Distress Syndrome in a patient. Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are still life-threatening diseases in critically ill patients. This online PaO2/FIO2 Ratio calculator assists you in calculating the PaO2:FiO2 ratio for the determination of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) in a patient. If PaO2:FiO2 < 300 mmHg (40 kPa) acute lung injury (ALI) is considered to be present. If PaO2:FiO2 < 200 mmHg (26.7 kPa) acute respiratory distress syndrome (ARDS) is considered to be present.