Aaron Graves, PhDude (Replica)

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pao2 fio2 calculator

Posted on February 16, 2026 by Admin

PaO2/FiO2 Ratio Calculator

Use this tool to calculate the P/F ratio (PaO2/FiO2) for oxygenation assessment in hypoxemia and ARDS screening.

Tip: Enter either 0.21-1.00 or 21-100.

What is the PaO2/FiO2 ratio?

The PaO2/FiO2 ratio (often called the P/F ratio) is a fast way to estimate how well oxygen moves from the lungs into the blood. It combines:

  • PaO2: arterial oxygen pressure from an ABG, measured in mmHg
  • FiO2: inspired oxygen fraction, written as a decimal (room air = 0.21)

Clinicians use this metric in critical care, emergency medicine, and respiratory therapy to track oxygenation trends and to help classify ARDS severity when paired with other clinical findings.

Formula and unit conversion

The equation

P/F ratio = PaO2 / FiO2

Example: PaO2 of 80 mmHg on FiO2 0.40 gives 80 / 0.40 = 200.

Common FiO2 input mistakes

The biggest error is entering FiO2 as a percent but calculating as if it were a fraction. This calculator handles both styles:

  • If you type 40, it is interpreted as 40% and converted to 0.40.
  • If you type 0.40, it is used directly.

How to interpret your P/F ratio

Interpretation depends on context, ventilator settings, timing, imaging, and etiology. As a quick bedside framework:

  • > 400: usually near-normal oxygenation in many situations
  • 300-400: mild impairment possible
  • 200-300: concerning for mild oxygenation deficit; can align with mild ARDS range if full criteria are met
  • 100-200: moderate oxygenation deficit; may align with moderate ARDS range
  • <= 100: severe oxygenation deficit; may align with severe ARDS range
ARDS diagnosis is not based on P/F ratio alone. Berlin criteria also include timing, bilateral opacities, and exclusion of cardiogenic edema. PEEP/CPAP level (typically at least 5 cm H2O) also matters.

Worked examples

Example 1: Room air

PaO2 92 on FiO2 0.21 gives a ratio around 438, which is generally reassuring.

Example 2: Supplemental oxygen

PaO2 85 on FiO2 0.50 gives a ratio of 170, indicating significantly impaired oxygenation.

Example 3: Mechanical ventilation context

PaO2 70 on FiO2 0.70 gives a ratio of 100. If other criteria are met and PEEP is adequate, this can be in severe ARDS territory.

Clinical caveats you should know

  • PEEP changes the picture: A P/F ratio should be interpreted with current ventilator settings.
  • Trend over time: One data point is less useful than serial values.
  • Altitude effects: Baseline oxygenation differs with barometric pressure.
  • Underlying lung disease: Chronic conditions may alter expected values.
  • Sampling quality: ABG errors or delays can distort interpretation.

How to use this calculator effectively

  • Use the most recent ABG PaO2 value.
  • Enter the exact FiO2 at the time of ABG draw.
  • Add PEEP when available for better ARDS context.
  • Compare with prior P/F values to identify improvement or deterioration.
  • Always integrate with patient exam, imaging, and hemodynamics.

FAQ

Can I enter FiO2 as 21, 40, or 100?

Yes. The calculator converts values greater than 1 and up to 100 into fractions automatically.

Does this replace clinical judgment?

No. This is a support tool only. Final decisions should be made by qualified clinicians using complete clinical data.

Is this the same as oxygenation index?

No. Oxygenation index includes mean airway pressure and is a different metric, often used in ventilated patients.

Bottom line

The PaO2/FiO2 ratio is a simple and useful oxygenation metric. Used correctly, it helps clinicians quickly quantify hypoxemia, communicate severity, and monitor treatment response.

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