P/F Ratio Calculator

Calculate the PaO2/FiO2 ratio, a critical indicator of lung function used in assessing acute respiratory distress syndrome (ARDS) severity.

How to Use This Calculator

Enter the arterial oxygen tension (PaO₂) value in mmHg from an ABG analysis
Enter the fraction of inspired oxygen (FiO₂) as a percentage (e.g., 50 for 50%)
Review your inputs for accuracy
Click Calculate to see the P/F ratio and ARDS severity classification

Formula Used

P/F Ratio = PaO₂ (mmHg) ÷ FiO₂ (as decimal)

Where:

PaO₂ = Arterial oxygen tension (in mmHg)
FiO₂ = Fraction of inspired oxygen (as decimal, e.g., 0.5 for 50%)

Example Calculation

Real-World Scenario:

A patient in the ICU is receiving 50% oxygen through a ventilator. An arterial blood gas analysis shows a PaO₂ of 85 mmHg.

Given:

PaO₂ = 85 mmHg
FiO₂ = 50% (0.5 as decimal)

Calculation:

P/F Ratio = 85 mmHg ÷ 0.5 = 170

Result: P/F Ratio of 170, which falls in the moderate ARDS category (100-200)

Why This Calculation Matters

Practical Applications

Diagnosing and classifying ARDS severity
Guiding ventilator management strategies
Predicting patient outcomes and mortality risk

Key Benefits

Standardized assessment of lung injury severity
Helps determine appropriate PEEP levels
Assists in clinical decision-making for respiratory support

Common Mistakes & Tips

Always enter FiO₂ as a percentage (e.g., 50 for 50%) rather than as a decimal (0.5). The calculator will automatically convert it to decimal for the calculation. Remember that room air is approximately 21% oxygen.

The P/F ratio should be calculated after the patient has been on a stable FiO₂ setting for at least 30 minutes. Taking an ABG sample too soon after changing oxygen settings may not reflect the true oxygenation status.

Frequently Asked Questions

According to the Berlin Definition, ARDS is classified as:

Mild: P/F ratio > 200 to ≤ 300
Moderate: P/F ratio > 100 to ≤ 200
Severe: P/F ratio ≤ 100

Yes, the P/F ratio can be calculated for patients on high-flow nasal cannula (HFNC) by using the approximate FiO₂ delivered by the device. However, the actual FiO₂ delivered by HFNC may vary based on flow rate and patient breathing patterns, so the calculated P/F ratio should be interpreted with caution.

The frequency of P/F ratio calculation depends on the clinical situation. In critically ill patients with ARDS, it may be calculated every 4-6 hours or after significant changes in ventilator settings. For stable patients, it may be calculated once daily or less frequently.

References & Disclaimer

Medical Disclaimer

This calculator is for educational purposes only and should not replace professional medical judgment. The P/F ratio is just one component of clinical assessment and should be interpreted in the context of the complete clinical picture.

References

The Berlin Definition of ARDS - ARDS Definition Task Force, JAMA 2012
Acute Respiratory Distress Syndrome - Thompson et al., New England Journal of Medicine 2017
Surviving Sepsis Campaign Guidelines - Society of Critical Care Medicine 2020

Accuracy Notice

The P/F ratio calculation assumes accurate measurement of PaO₂ and FiO₂. In clinical practice, factors such as altitude, patient position, and timing of blood gas sampling can affect the accuracy of this ratio. This calculator does not account for all clinical variables that may affect oxygenation status.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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