Calculate the P/F ratio as a bedside oxygenation measure and review it alongside Berlin ARDS severity bands, A-a gradient, oxygenation index, and estimated S/F ratio.
The **PaO₂/FiO₂ ratio** (P/F ratio) is a common bedside index of oxygenation efficiency. It standardizes arterial oxygen tension against the fraction of inspired oxygen so oxygenation can be compared at different FiO₂ levels. A normal P/F ratio is often around 400–500 (for example, PaO₂ 95 mmHg on room air gives about 452). Values below 300 suggest impaired oxygenation, and the **Berlin 2012 definition of ARDS** uses the ratio as one of its main severity criteria.
The Berlin criteria classify ARDS into three severities: **mild** (P/F 200–300), **moderate** (P/F 100–200), and **severe** (P/F < 100), all in the setting of PEEP ≥ 5 cmH₂O and compatible clinical and radiographic findings. The ratio helps organize severity, but it does not diagnose ARDS by itself and should not be interpreted without the broader respiratory context.
Beyond the P/F ratio, this calculator provides complementary oxygenation metrics. The **A-a gradient** (alveolar-arterial oxygen difference) helps frame whether hypoxemia looks more like hypoventilation or a gas-exchange problem. The **oxygenation index** (OI = FiO₂ × MAP × 100 / PaO₂) adds ventilator-support context. The **S/F ratio** (SpO₂/FiO₂) is shown as a rough non-invasive approximation rather than a substitute for an arterial blood gas.
The P/F ratio is a practical way to summarize oxygenation impairment, but it becomes more useful when it is paired with the A-a gradient and oxygenation index instead of being viewed alone. This calculator keeps those values together so the severity label and the physiologic context can be reviewed in one place.
P/F Ratio = PaO₂ / FiO₂ (decimal). A-a Gradient = PAO₂ − PaO₂, where PAO₂ = FiO₂(Patm − 47) − PaCO₂/0.8. Oxygenation Index = (FiO₂ × MAP × 100) / PaO₂. Normal A-a gradient ≈ Age/4 + 4.
Result: P/F = 117 — Moderate ARDS
PaO₂ 70 / FiO₂ 0.60 = 117. With PEEP ≥ 5, this falls in the moderate ARDS range (100–200). OI = (0.60 × 18 × 100)/70 = 15.4, indicating severe oxygenation impairment.
The same PaO2/FiO2 value means something different on room air than it does on high ventilator support. That is why the ratio is most helpful when PEEP, FiO2, and mean airway pressure are considered at the same time rather than as separate numbers.
The P/F ratio is good for severity classification, but the A-a gradient and oxygenation index add information about mechanism and ventilator burden. Together, they help distinguish poor oxygenation from poor ventilation strategy or inadequate support.
One value gives a snapshot. A sequence of values over hours to days is more useful for judging response to therapy, especially when prone positioning, PEEP changes, or ECMO consideration are on the table.
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This page calculates the PaO₂/FiO₂ ratio by dividing arterial oxygen tension by inspired oxygen fraction expressed as a decimal, then places the result next to the Berlin ARDS severity bands when the entered PEEP meets the 5 cmH₂O threshold used in the definition. It also computes the alveolar oxygen estimate and A-a gradient from FiO₂, barometric pressure, and PaCO₂, and it adds an oxygenation index based on mean airway pressure so oxygenation can be reviewed with some ventilator-support context. The S/F ratio shown on the page is only a rough noninvasive approximation.
The page is an oxygenation worksheet, not an ARDS diagnosis or rescue-therapy protocol. ARDS still requires the broader Berlin clinical and imaging criteria, and ventilator changes, prone positioning, ECMO decisions, and other treatment steps depend on the full ICU picture rather than on the ratio alone.
Normal P/F is 400–500. On room air (FiO₂ 0.21), a PaO₂ of 95 gives P/F = 452. Values below 300 suggest oxygenation impairment.
The S/F ratio (SpO₂/FiO₂) correlates with P/F: an S/F of 235 ≈ P/F of 200, and S/F of 315 ≈ P/F of 300. It is useful for screening, but arterial blood gas remains the direct measurement for PaO₂-based calculations.
The Berlin definition requires a minimum PEEP of 5 cmH₂O because without PEEP, many patients with atelectasis would be misclassified as ARDS. PEEP helps ensure the P/F reduction reflects true parenchymal disease.
The A-a gradient helps separate hypoxemia caused mainly by hypoventilation from hypoxemia caused by V/Q mismatch, shunt, or diffusion impairment. In other words, a normal gradient points more toward low ventilation, while an elevated gradient suggests a gas-exchange problem in the lungs.
ECMO decisions depend on the full clinical picture, local expertise, and rescue-therapy response rather than on one number alone. Very low P/F ratios or a high oxygenation index can signal the need for urgent specialist review, but this calculator should not be used as a stand-alone ECMO threshold tool.
Yes. At altitude, barometric pressure decreases, lowering the PAO₂ and thus PaO₂. Berlin criteria should be adjusted at altitude, or the A-a gradient used instead.