The PF ratio divides arterial oxygen by the fraction of inspired oxygen to grade how severely the lungs are failing at oxygenation, and is the core metric used to diagnose and stage ARDS.
Normal to at-risk oxygenation.
Mild ARDS (Berlin criteria).
Moderate ARDS.
Severe ARDS.
Given: PaO₂ 80 mmHg on FiO₂ 0.5.
160Lower ratios mean worse oxygenation — the Berlin criteria set the severity bands.
Yes — the PaO₂ must come from an arterial blood gas, not pulse oximetry. The non-invasive SpO₂/FiO₂ ratio is a separate surrogate measure that some protocols use when an ABG isn't available.
Greater than about 400. By the Berlin criteria for ARDS, 200–300 is mild, 100–200 is moderate, and 100 or below is severe.
FiO₂ is entered as a fraction (0.21–1.00), so PaO₂ 80 on FiO₂ 0.5 gives 80 ÷ 0.5 = 160. Entering FiO₂ as a percent by mistake makes the ratio 100× too small.
The Berlin ARDS definition requires the PF ratio to be measured on at least 5 cmH₂O of PEEP, because PEEP itself changes oxygenation. This calculator computes the raw ratio; interpret it in that context.