P/F ratio calculator (PaO2/FiO2)
The P/F ratio (P to F ratio) is the arterial PaO2 in mmHg divided by FiO2 as a fraction. For example, a PaO2 of 80 mmHg on FiO2 0.40 is a P/F of 200. In the Berlin definition of ARDS, with PEEP of at least 5 cm H2O, over 200 to 300 is mild, over 100 to 200 moderate, and 100 or less severe.
Clinical disclaimer
Use FiO2 as a fraction (0.21-1.0), not percent.
Use an arterial blood gas value; kPa is converted to mmHg.
P/F ratio calculator quick read
The P/F ratio is calculated as PaO2 (mmHg) divided by FiO2 (fraction). It is often used to describe oxygenation impairment:
- P/F ratio = PaO2 / FiO2.
- FiO2 must be entered as a fraction (0.21-1.0).
- PaO2 may be entered in mmHg or kPa (converted to mmHg).
Interpretation and pediatric context
| P/F ratio | Berlin ARDS (adults, PEEP 5+; CPAP counts for mild only) | PALICC-2 (children on full-face mask CPAP/BiPAP 5+) |
|---|---|---|
| Over 300 | Outside the ARDS range | Outside the PARDS P/F criterion |
| Over 200 to 300 | Mild | Mild/moderate NIV-PARDS |
| Over 100 to 200 | Moderate | Mild/moderate NIV-PARDS |
| 100 or less | Severe | Severe NIV-PARDS |
Both definitions also need the other criteria (timing, chest imaging, not explained by heart failure or fluid overload); the ratio alone is not a diagnosis. For children on invasive ventilation, PALICC-2 uses the oxygenation index (OI 4 or more, or OSI 5 or more; severe at OI 16 or OSI 12) instead of the P/F ratio.
PALICC-2 also stages severity only after at least 4 hours, treats nasal CPAP or high flow as possible PARDS, and for children with cyanotic heart disease or chronic lung disease requires an acute drop in oxygenation from their baseline rather than a fixed cutoff.
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References
- 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.
- Emeriaud G, Lopez-Fernandez YM, Iyer NP, et al. Executive summary of the Second International Guidelines for the Diagnosis and Management of Pediatric Acute Respiratory Distress Syndrome (PALICC-2). Pediatr Crit Care Med. 2023;24(2):143-168.
Frequently asked questions
The PaO2/FiO2 ratio summarizes oxygenation efficiency by comparing arterial oxygen (PaO2) to the fraction of inspired oxygen (FiO2). Lower values suggest worse oxygenation.
No. The P/F ratio is defined using arterial PaO2 from an ABG. If you only have SpO2, consider an SpO2/FiO2 (S/F) ratio or OSI instead.
There is no single normal cutoff in the guidelines. A P/F above 300 is outside the Berlin ARDS range; 300 or less, with PEEP or CPAP of at least 5 cm H2O and the other Berlin criteria, meets the ARDS oxygenation criterion.
Yes, for children on full-face mask CPAP or BiPAP of at least 5 cm H2O. PALICC-2 (2023) uses a P/F of 300 or less (or S/F of 250 or less) with the other PARDS criteria, and calls a P/F of 100 or less severe; on nasal CPAP or high flow the same values mean possible PARDS. Children on invasive ventilation are diagnosed and staged by oxygenation index (OI) or oxygen saturation index (OSI).
Choose kPa and the calculator converts to mmHg before dividing. For example, a PaO2 of 10 kPa on FiO2 0.5 is about 75 mmHg, a P/F of 150.