P/F Ratio Calculator & Oxygenation Index Guidance
Calculate the P/F ratio and oxygenation index (OI) to assess oxygenation failure, estimate ARDS severity, and understand when a patient may be approaching ECMO-level respiratory failure. This tool is built for respiratory therapists, ICU clinicians, and students who want a fast bedside framework for interpreting oxygenation.
For deeper interpretation, pair this calculator with our P/F Ratio guide, A–a Gradient guide, Causes of Hypoxemia, and Mastering ABG Analysis.
What this calculator tells you
P/F ratio
The PaO₂/FiO₂ ratio helps grade oxygenation severity and is a core part of ARDS classification.
Oxygenation index
The oxygenation index adds mean airway pressure, making it more useful when ventilator support intensity matters.
Quick interpretation guide
P/F ratio cutoffs
- Normal: > 300
- Mild ARDS: 201–300
- Moderate ARDS: 101–200
- Severe ARDS: ≤ 100
OI checkpoints
- Lower values suggest milder oxygenation impairment
- OI ≥ 25 suggests severe oxygenation failure
- OI ≥ 40 raises concern for ECMO-level severity
This tool works best when combined with broader gas-exchange assessment using the causes of hypoxemia framework and the A–a gradient.