What is the P/F ratio?

The P/F ratio is arterial oxygen tension (PaO₂) divided by the inspired oxygen fraction (FiO₂ expressed as a decimal). It summarises how efficiently the lungs are oxygenating blood, with lower values indicating worse gas exchange.

How does the Berlin definition classify ARDS?

With PEEP of at least 5 cmH₂O, a P/F ratio of 200 to 300 is mild ARDS, 100 to 200 is moderate, and below 100 is severe. A ratio above 300 does not meet the oxygenation criterion for ARDS.

Do I enter FiO₂ as a percent or a fraction?

Enter it as a percent, such as 60 for 60 percent oxygen. The tool converts it to the 0.60 fraction internally before dividing. Room air is 21 percent, which is the minimum the tool accepts.

My blood gas reports PaO₂ in kPa — does that matter?

Yes. The Berlin thresholds are defined in mmHg, so the tool converts kPa to mmHg using 1 kPa equals 7.5 mmHg before computing the ratio. Just select kPa as the unit and enter your value.

Is the P/F ratio enough to diagnose ARDS?

No. The Berlin definition also requires acute onset within a week, bilateral infiltrates on imaging not fully explained by cardiac failure or fluid overload, and a minimum PEEP. The ratio supplies only the oxygenation severity grade.

P/F Ratio (PaO₂/FiO₂) Calculator — Berlin ARDS Criteria

The P/F ratio — arterial oxygen tension divided by inspired oxygen fraction — is the single most used number for grading hypoxaemic respiratory failure. This calculator computes it from a blood gas and ventilator setting, then classifies severity under the Berlin definition of acute respiratory distress syndrome, all in your browser.

How it works

The ratio divides PaO₂ (in mmHg) by FiO₂ expressed as a fraction:

P/F = PaO2_mmHg / (FiO2_percent / 100)

If your gas reports PaO₂ in kPa, the tool first converts it (1 kPa = 7.5 mmHg) because the Berlin thresholds are defined in mmHg. The Berlin definition, assuming PEEP of at least 5 cmH₂O, then grades the result: 200–300 is mild ARDS, 100–200 moderate, and below 100 severe; above 300 does not meet the oxygenation criterion.

Example and notes

A patient with a PaO₂ of 80 mmHg on 60% oxygen has a P/F ratio of 80 / 0.60 = 133, placing them in the moderate ARDS band. The ratio is sensitive to FiO₂, so a patient can move between categories simply by changing the oxygen setting — interpret it alongside PEEP and the ventilator strategy. The oxygenation grade is only one of the Berlin criteria; acute onset, bilateral infiltrates, and a non-cardiac cause must also be present to diagnose ARDS.