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P/F ratio calculator (PaOâ‚‚/FiOâ‚‚)

Study aid only. Not for clinical decisions. This calculator teaches textbook formulas for exam prep. It is not a medical device and does not replace clinical judgment, your facility's protocols, or a licensed clinician's assessment. Nothing you enter leaves your browser.

Formula

P/F = PaO2 ÷ FiO2 (FiO2 as a decimal). Berlin ARDS oxygenation categories, measured on PEEP or CPAP of at least 5 cm H2O:[1]

CategoryPaO2/FiO2 (mm Hg)
Mild> 200 to ≤ 300
Moderate> 100 to ≤ 200
Severe≤ 100

The oxygenation criterion is only one part of the Berlin definition. The others are timing within 1 week, bilateral opacities on imaging, and respiratory failure not fully explained by heart failure or fluid overload.[1]

Worked example

PaO2 80 mm Hg on FiO2 0.40 with PEEP 10: 80 ÷ 0.4 = 200, which is moderate on the Berlin scale.

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Sources

  1. ARDS Definition Task Force. The Berlin Definition. JAMA 2012
  2. StatPearls: Arterial Blood Gas

Formulas checked against the sources listed above on October 30, 2026. Written and reviewed by a practicing RRT for TidalPrep.