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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.

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PaO2 (mmHg)

FiO2 (%)

Result

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Formula

Core Formula

\text{P/F Ratio} = \frac{\text{PaO}_2}{\text{FiO}_2}

How it works: Provides a standardized measure of oxygenation regardless of how much supplemental oxygen the patient is receiving.

Worked Example

PaO2 60 on 60% FiO2 (0.6) -> P/F Ratio 100 (Severe ARDS severity range).

PF Ratio Calculator - PaO2/FiO2 Formula and ARDS Staging

The PaO2/FiO2 ratio (P/F ratio) is the cornerstone of the Berlin Definition for classifying Acute Respiratory Distress Syndrome. It provides a standardized way to assess oxygenation severity regardless of supplemental oxygen levels.

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.

PaO2/FiO2 Ratio: ARDS Severity Classification (Berlin Criteria)

Acute Respiratory Distress Syndrome severity staging based on the Berlin Definition. Requires bilateral opacities on imaging and PEEP of 5 cmH2O or greater.

PF Ratio	Severity	Mortality Rate	Typical Ventilator Strategy
Greater than 400	Normal	N/A	No ARDS
300-400	At risk	N/A	Monitor, supplemental oxygen
200-299	Mild ARDS	27%	Low tidal volume ventilation (6 mL/kg IBW)
100-199	Moderate ARDS	32%	Low TV, consider prone positioning
Less than 100	Severe ARDS	45%	Prone positioning, consider ECMO, neuromuscular blockade

Note: Berlin criteria require: acute onset (within 1 week), bilateral opacities on chest imaging not fully explained by effusions or atelectasis, respiratory failure not fully explained by cardiac failure, and PEEP of 5 cmH2O or greater. PF ratio must be measured while on invasive or non-invasive ventilation. For educational reference only.

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.

Can I embed this PaO2/FiO2 Ratio Calculator on my website?

Yes. Click the "Embed" button at the top of this page to customize the size, colors, and theme, then copy the iframe code. Paste it into any HTML page, WordPress site, or CMS. It is completely free, requires no signup, and works on all devices. You can also visit our embed guide at calculory.com/services/embed-calculators for more details.

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.

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