π« PaO2/FiO2 Ratio Calculator
Grade ARDS severity with the Berlin definition.
For informational purposes only, not medical advice. Consult a healthcare professional.
The PaO2 to FiO2 ratio, universally called the P/F ratio, is the single number intensivists quote when describing how badly lungs are failing. It divides the oxygen in arterial blood by the fraction of oxygen being breathed, so it adjusts for the fact that a PaO2 of 90 means very different things on room air versus 100 percent oxygen. The 2012 Berlin definition of ARDS grades severity entirely on this ratio: 200 to 300 is mild, 100 to 200 is moderate, and 100 or below is severe, each with its own expected mortality.
How to Use This Calculator
Enter the PaO2 from the arterial blood gas and the FiO2 as a percent, plus the PEEP level so the calculator can check the Berlin definition requirements.
Take the PaO2 in mmHg from a recent arterial blood gas and the FiO2 straight from the ventilator or oxygen device settings.
Enter the PEEP in cmH2O. The Berlin definition requires PEEP of at least 5 for an ARDS diagnosis, which the calculator verifies.
Read the P/F ratio and the severity grade: mild, moderate, or severe ARDS by Berlin cutoffs, with the mortality context for each.
How the Math Works
Divide PaO2 in mmHg by FiO2 as a decimal fraction. A PaO2 of 90 on 60 percent oxygen gives 90 divided by 0.6, which is 150. The Berlin definition requires the measurement on PEEP of at least 5 cmH2O, onset within one week of a known trigger, bilateral opacities on imaging not fully explained by cardiac failure, and respiratory failure not fully explained by fluid overload. The severity cutoffs carry the mortality data from the Berlin validation cohorts: roughly 27 percent for mild, 32 percent for moderate, and 45 percent for severe ARDS. This calculator applies the ratio cutoffs and flags the PEEP requirement.
Worked Example
A patient with PaO2 90 mmHg on 60 percent oxygen with PEEP 8 has a P/F ratio of 150. The calculator grades this as Moderate ARDS by the Berlin cutoffs, since 150 falls in the 100 to 200 band and the PEEP requirement is satisfied. The same PaO2 on room air would give a ratio near 430, which shows why the ratio, not the raw PaO2, is the number that matters.
Reading Your Result
Above 300 on meaningful support suggests no ARDS by this criterion, though the patient may still be quite ill. The 200 to 300 mild band warrants lung-protective ventilation and a search for the underlying cause. The 100 to 200 moderate band is where proning and higher PEEP strategies enter guidelines. At 100 or below, severe ARDS, mortality approaches half and rescue therapies including ECMO evaluation belong in the discussion. The ratio is one pillar of the Berlin definition; imaging and timing criteria must also be met before calling it ARDS.
| P/F ratio | Berlin grade | Validation mortality |
|---|---|---|
| 200 to 300 | Mild ARDS | About 27%% |
| 100 to 200 | Moderate ARDS | About 32%% |
| 100 or less | Severe ARDS | About 45%% |
Limitations
The ratio is a snapshot that swings with suctioning, positioning, and ventilator changes, so trends beat single values. It does not distinguish ARDS from cardiogenic edema, pneumonia alone, or atelectasis; the Berlin imaging and timing criteria exist for that. Altitude lowers PaO2 and shifts the ratio, and the cutoffs were validated at sea level in ventilated adults.
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For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is a normal P/F ratio?
On room air at sea level, roughly 400 to 500: a PaO2 near 90 to 100 divided by 0.21. Values above 300 are generally considered outside ARDS territory. The ratio falls as lung injury worsens or as more oxygen is needed to hold the same PaO2.
Why divide by FiO2 instead of just using PaO2?
Because PaO2 without context lies. A PaO2 of 85 looks fine until you learn the patient needs 100 percent oxygen to hold it, which gives a P/F of 85, severe ARDS. The same 85 on room air gives 405, essentially normal. The ratio exposes how much support the oxygen level costs.
What is the Berlin definition of ARDS?
The 2012 consensus definition: acute onset within one week of a trigger, bilateral opacities on chest imaging not fully explained by effusions or collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and hypoxemia graded by P/F ratio on PEEP of at least 5. All four pillars are needed for the diagnosis.
Can I use SpO2 instead of PaO2?
There is a validated substitute, the S/F ratio using SpO2, with its own cutoffs, but it is a different number and this calculator does not compute it. SpO2 plateaus at 100 percent and hides the true PaO2, so the P/F ratio specifically requires an arterial blood gas.
Does PEEP change the ratio?
Yes, and that is why Berlin requires PEEP of at least 5. Higher PEEP recruits collapsed lung and usually raises the ratio, which can make ARDS look milder than it is. Comparing ratios across different PEEP levels is misleading; note the PEEP alongside every value.
What P/F ratio suggests intubation?
No single cutoff mandates intubation; the decision weighs work of breathing, mental status, and trajectory. In practice, a P/F persistently under 150 to 200 on high-flow oxygen with distress pushes most clinicians toward intubation, and under 100 is severe failure where delay is dangerous.
How does altitude affect the ratio?
At altitude the inspired oxygen pressure is lower, so PaO2 falls even in healthy lungs and the ratio drops. The Berlin cutoffs were validated near sea level. At significant altitude, clinicians interpret the ratio with that offset in mind rather than applying sea-level cutoffs rigidly.
Is the P/F ratio the same as the oxygenation index?
No. The P/F ratio ignores ventilator pressures; the oxygenation index adds mean airway pressure to capture the pressure cost of oxygenation. Adults are usually described with P/F, while neonatal and pediatric ICUs favor the oxygenation index. Both describe the same underlying failure from different angles.
Can the ratio improve without the lungs healing?
Temporarily, yes. Proning, higher PEEP, or clearing a mucus plug can raise the ratio within minutes without changing the underlying injury. That is useful therapeutically but means a single improved value is not proof of recovery. Sustained improvement across days is what counts.
What comes after a severe ARDS grade?
Lung-protective ventilation with low tidal volumes, proning for 12-plus hours daily, conservative fluids, and evaluation for ECMO if the P/F stays under about 80 to 100 despite optimal care. Severe ARDS is managed in ICUs with these rescue options available, and early referral beats late referral.