P/F Ratio Calculator (PaO2/FiO2)
What is P/F Ratio Calculator (PaO2/FiO2)
The P/F ratio divides the arterial oxygen tension (PaO2, in mmHg) by the inspired oxygen fraction (FiO2). It gives a single number that summarizes how well the lungs are oxygenating the blood, independent of the oxygen the patient is receiving. It anchors the Berlin definition of ARDS: above 300 is not ARDS by this criterion, 201 to 300 is mild, 101 to 200 is moderate, and 100 or below is severe (with PEEP of at least 5 cmH2O).
How to use
- Enter the PaO2 from the arterial blood gas in mmHg.
- Enter the FiO2 as a percentage (for example 40) or a fraction (0.4).
- Read the ratio and the ARDS severity band that appears.
- Confirm PEEP or CPAP of at least 5 cmH2O before applying the Berlin grade.
When to use it
Grading hypoxemia on a ventilated patient and need the ARDS severity fast? Enter a PaO2 of 80 on 40% oxygen and you get 200, which lands in moderate ARDS. Useful on rounds, when charting a trajectory across blood gases, or when deciding whether proning and lung-protective settings are indicated.
Frequently asked questions
How do I enter FiO2 in this calculator?
Either way works. Type 40 for 40% or 0.4 as a fraction, and the calculator normalizes it. Room air is about 21%, or 0.21.
What P/F ratio defines ARDS?
By the Berlin definition, a ratio of 300 or below with PEEP or CPAP of at least 5 cmH2O suggests ARDS: 201 to 300 mild, 101 to 200 moderate, and 100 or below severe. The full diagnosis also needs bilateral infiltrates and a non-cardiac cause.
Does the P/F ratio change with altitude or units?
Yes. This tool assumes PaO2 in mmHg at sea level. At altitude the same lungs give a lower PaO2, so the ratio drops; if your gas reports kPa, convert to mmHg first (1 kPa is about 7.5 mmHg).
Can I share this calculator with the values pre-filled?
Yes. The URL updates as you type PaO2 and FiO2. Copy it from the address bar or use the Copy link button, and anyone who opens it sees the same P/F ratio ready to go.
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