P/F ratio calculator (PaO2/FiO2)
Calculate the P to F ratio from ABG PaO2 and FiO2, then interpret oxygenation trends with pediatric respiratory context.
Last reviewed: February 26, 2026
Clinical disclaimer
P/F ratio calculator quick read
- Use this P/F ratio calculator when you need the PaO2/FiO2 ratio from an ABG and matched FiO2.
- The formula is P/F ratio = PaO2 ÷ FiO2, with FiO2 entered as a decimal from 0.21 to 1.0.
- Trend the P to F ratio with ventilator settings and clinical trajectory rather than using one isolated number.
Use FiO2 as a fraction (0.21-1.0), not percent.
Use an arterial blood gas value; kPa is converted to mmHg.
How it works
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).
Rapid bedside workflow
- Confirm FiO2 and ABG PaO2 come from the same clinical timepoint.
- Calculate P/F ratio and compare against prior values instead of one isolated number.
- Interpret with ventilator settings, hemodynamics, and exam trajectory.
- Set next reassessment interval and escalation trigger if oxygenation worsens.
Interpretation and pediatric context
Adult ARDS criteria (Berlin definition) use P/F ratio thresholds to describe severity. Pediatric ARDS (PARDS) guidance often relies on oxygenation index (OI) or oxygen saturation index (OSI), especially in ventilated children.
- Use P/F ratio as a quick snapshot of oxygenation efficiency.
- Confirm ventilator settings and ABG timing when interpreting trends.
- For pediatric classification and escalation, consider OI/OSI and local protocols.
Reassessment cadence
- After major ventilator or oxygen change: recalculate with updated synchronized ABG data.
- During unstable hypoxemia: repeat trending at short intervals per local respiratory pathway.
- Any clinical deterioration despite stable ratio: immediate reassessment and escalation.
Shift handoff checklist
- Current ratio with FiO2, PaO2, and timestamp.
- Trend versus prior ratio and associated ventilator context.
- Interventions attempted and observed response.
- Next reassessment time and escalation threshold.
Related calculators
References
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.
Normal values are typically 300 or higher when FiO2 is reported correctly. Values below 300 indicate impaired oxygenation, but reference ranges vary by context.
Pediatric ARDS guidance often prefers oxygenation index (OI) or oxygen saturation index (OSI). The P/F ratio can still be helpful as a quick snapshot.
Document FiO2, PaO2 source/time, and trend direction with each value. Context-rich documentation prevents misinterpretation from mixed-timepoint data.