P/F Ratio Calculator
PaO₂ divided by FiO₂ — the fastest single number for trending oxygenation impairment.
Written by Apex Respiratory Editorial Team
Accepts 0.21–1.0 or 21–100.
Enter PaO₂ and FiO₂ to calculate the ratio.
Why P/F matters
The P/F ratio normalizes PaO₂ for how much oxygen the patient is receiving, which makes oxygenation comparable across FiO₂ changes. A healthy adult on room air runs roughly 400–500.
The Berlin definition bands (mild 200–300, moderate 100–200, severe ≤ 100) apply with PEEP or CPAP of at least 5 cmH₂O and an appropriate clinical picture — the ratio alone never diagnoses ARDS.
Educational use only. Severity bands assume PEEP/CPAP ≥ 5 cmH₂O per the Berlin definition. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526-2533.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.