PaO₂ / FiO₂ Ratio

What This Calculator Measures

Calculates the PaO₂/FiO₂ (P/F) ratio to quantify the degree of oxygenation impairment and to support classification of ARDS severity according to the Berlin Definition.

How to Use

1. Enter the arterial oxygen tension (PaO₂) in mmHg and the fraction of inspired oxygen (FiO₂) as a fraction (room air = 0.21).
2. Use PaO₂ and FiO₂ measured concurrently, ideally from the same arterial blood gas.
3. Recognize that formal Berlin ARDS classification requires the appropriate clinical context, including timing, bilateral opacities, and the required PEEP.

How the Calculation Works

P/F ratio = PaO2 / FiO2. PaO2 is the arterial partial pressure of oxygen in mmHg and FiO2 is the inspired oxygen fraction (0.21 on room air). Lower ratios indicate worse oxygenation and are used to grade ARDS severity.

Variables Explained

Interpretation Guide

P/F >400 indicates normal oxygenation; 301–400 mild oxygenation impairment; 201–300 mild ARDS; 101–200 moderate ARDS; ≤100 severe ARDS. The ARDS bands apply when the broader Berlin criteria, including the appropriate PEEP and clinical context, are met.

Clinical Significance

The P/F ratio is a standard bedside measure of oxygenation and, within the full Berlin Definition framework, is used to grade ARDS severity and guide the intensity of respiratory support.

When It Is Useful

• Quantifying the degree of oxygenation impairment in hypoxemic patients
• Supporting ARDS severity classification within the appropriate clinical context
• Monitoring oxygenation trends in critically ill patients

When It Is Not Useful

• As the sole criterion for an ARDS diagnosis, without the full Berlin Definition context
• When PaO₂ and FiO₂ are not measured concurrently
• In children, where pediatric-specific ARDS (PARDS) criteria apply

Common Mistakes

• Entering FiO2 as a percentage instead of a fraction, such as entering 100 instead of 1 for pure oxygen, which produces a falsely low ratio.
• Combining a PaO2 from one blood gas with an FiO2 recorded at a different time, since the ratio is only meaningful when both values are measured concurrently.
• Applying the Berlin ARDS severity bands to a patient who lacks bilateral infiltrates or a PEEP of at least 5 cmH2O, misclassifying simple hypoxemia as ARDS.

Limitations

• The P/F ratio varies with FiO₂, so values obtained at different FiO₂ levels are not directly comparable.
• Formal Berlin ARDS classification also requires timing of onset, bilateral infiltrates, and a PEEP of at least 5 cmH₂O, which this calculator does not capture.
• The Berlin Definition applies to adults; pediatric ARDS is classified separately.
• The P/F ratio alone does not identify the cause of hypoxemia.

Worked Example

A 62-year-old woman with pneumonia is intubated on FiO₂ 0.4 with a concurrent PaO₂ of 110 mmHg.

Expected Result: P/F ratio = 110 / 0.4 = 275 — mild ARDS (Berlin Definition band 201–300, with appropriate PEEP).

Evidence & References

Source:

Berlin Definition Consensus

Reference:

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.

Content updated:

2026-08

1. ARDS Definition Task Force; Ranieri VM, et al. JAMA. 2012;307(23):2526-2533.

References

1. ARDS Definition Task Force; Ranieri VM, et al. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526-2533. (Original classification)

Frequently Asked Questions