What this calculator does
An arterial oxygen level means nothing without knowing how much oxygen the patient was breathing. The PaO2/FiO2 ratio divides one by the other, producing a figure comparable across very different oxygen deliveries.
The bands are how ARDS severity is defined. The same PaO2 of 90 mmHg gives a ratio of 429 on room air, which is unremarkable, and 90 on pure oxygen, which falls in the severe range.
The formula
Divide arterial PaO2 in mmHg by the fraction of inspired oxygen expressed as a decimal. Room air is 0.21, so a PaO2 of 90 on room air gives 429.
| Term | Meaning |
|---|---|
| PaO2 | Partial pressure of oxygen in arterial blood, from a blood gas. |
| FiO2 | Fraction of inspired oxygen, from 0.21 on room air to 1.0 on pure oxygen. |
| Berlin criteria | The 2012 ARDS definition, which uses this ratio for severity grading. |
The inputs explained
| Field | What to enter |
|---|---|
| Arterial PaO₂ (mmHg) | Arterial PaO2 in mmHg from an arterial blood gas. |
| FiO₂ (%) | Fraction of inspired oxygen as a percentage. Room air is 21. |
When to use it
Grading respiratory failure severity
The ratio is the oxygenation component of the Berlin ARDS definition.
Tracking a patient over time
It shows whether oxygenation is improving even as the delivered oxygen changes.
Comparing across oxygen deliveries
Raw PaO2 is not comparable between room air and mechanical ventilation.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does inspired oxygen change the ratio?
The same arterial oxygen at four inspired fractions.
| FiO2 | PaO₂/FiO₂ ratio | Reading |
|---|---|---|
| 21% | 429 | Above 300: no significant oxygenation impairment by the Berlin ARDS criteria |
| 40% | 225 | 200 - 300: mild impairment (mild ARDS range) |
| 60% | 150 | 100 - 200: moderate impairment (moderate ARDS range) |
| 100% | 90 | 100 or below: severe impairment (severe ARDS range) |
Questions
Why is the same PaO2 read so differently?
Because the ratio measures efficiency, not the oxygen level itself. Achieving a PaO2 of 90 while breathing room air indicates healthy gas exchange; needing pure oxygen for the same result indicates it is badly impaired.
What are the Berlin severity bands?
Above 300 is not ARDS by the oxygenation criterion, 200 to 300 is mild, 100 to 200 moderate and 100 or below severe. The full definition also requires timing, imaging and an origin criterion.
Does PEEP affect the ratio?
Yes, substantially. The Berlin criteria require at least 5 cmH2O of PEEP precisely because the ratio improves with higher pressures, so an uncontrolled comparison can mislead.
Does altitude matter?
It does. The ratio was defined at sea level, and at altitude the lower barometric pressure reduces PaO2 for the same lung function. Corrections exist for use in high-altitude settings.
For the gas exchange gradient, see the A-a gradient calculator. For airflow obstruction, see the FEV1/FVC ratio calculator.