PaO2/FiO2 Ratio Calculator
Calculate the PaO2/FiO2 ratio (P/F Ratio) to classify respiratory failure severity. Normal P/F ratio is above 400. Under 300 indicates ALI or mild ARDS, under 200 is moderate ARDS, and under 100 is severe ARDS per the Berlin criteria.
Enter Values
Result
Enter values above and click Calculate to see your result.
AI Assistant
Ask about this calculator
I can help you understand the pao2/fio2 ratio calculator formula, interpret your results, and answer follow-up questions.
Try asking
Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.
Formula
Provides a standardized measure of oxygenation regardless of how much supplemental oxygen the patient is receiving.
Worked Example
P/F Ratio and ARDS Classification
- Normal P/F ratio: above 400 (e.g., PaO2 95 on room air = 452)
- Mild ARDS: P/F 200 to 300 with PEEP at least 5 cmH2O
- Moderate ARDS: P/F 100 to 200 with PEEP at least 5 cmH2O
- Severe ARDS: P/F below 100 with PEEP at least 5 cmH2O
- Mortality increases significantly with each severity category: mild ~27%, moderate ~32%, severe ~45%
The P/F ratio should be interpreted alongside clinical context, chest imaging, and PEEP settings. Serial measurements help track response to interventions like prone positioning and lung-protective ventilation.
You can also calculate changes using our A-a Gradient Calculator or Oxygen Requirement Calculator.
Frequently Asked Questions
What is a normal P/F ratio?
A normal P/F ratio is above 400. For example, a PaO2 of 95 mmHg on room air (FiO2 0.21) gives a P/F ratio of approximately 452.
What are the ARDS categories by P/F ratio?
Per the Berlin criteria: Mild ARDS is 200 to 300, Moderate ARDS is 100 to 200, and Severe ARDS is below 100. All require a minimum PEEP of 5 cmH2O.
Why use the P/F ratio instead of just PaO2?
Because PaO2 of 80 is normal on room air, but extremely poor if the patient is on 100% oxygen. The P/F ratio standardizes oxygenation across different FiO2 levels.
How do I convert FiO2 percentage to a decimal?
Divide the percentage by 100. For example, 40% FiO2 becomes 0.40. Room air (21%) becomes 0.21. The P/F ratio uses the decimal form in the denominator.
What is the Berlin Definition of ARDS?
The 2012 Berlin Definition classifies ARDS by P/F ratio on PEEP of at least 5 cmH2O, with bilateral opacities on imaging not fully explained by effusions or atelectasis, and respiratory failure not fully explained by cardiac failure or fluid overload.
Can the P/F ratio track treatment response?
Yes. Serial P/F ratios help monitor whether interventions like prone positioning, PEEP adjustments, or lung-protective ventilation are improving oxygenation over time.
Medical Disclaimer
This calculator is provided for informational and educational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions based on these results. Do not disregard or delay seeking medical advice because of information obtained from this tool.
AI Assistant
Ask about this calculator
I can help you understand the pao2/fio2 ratio calculator formula, interpret your results, and answer follow-up questions.
Try asking
Our AI assistant provides general estimates and advice only. Please consult a qualified healthcare professional before making any health-related decisions.
Related Calculators
Secure and Private
All calculations run locally. Your data never leaves your browser.
Precise Calculations Powered by Calculory AI