Respiratory

P/F Ratio (PaO₂/FiO₂) Calculator

PaO₂ divided by FiO₂, the oxygenation measure that grades ARDS and triggers proning.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

PaO₂/FiO₂ = PaO₂ / (FiO₂ as decimal)

How it is used

• Breathing room air, a healthy adult runs above 400.
• ARDS severity on PEEP 5 or more (Berlin): mild 201–300, moderate 101–200, severe 100 or less.
• P/F alone is not ARDS: the onset must be within a week, with bilateral opacities not explained by heart failure or fluid overload.

Key points

• A P/F ratio < 150 is often used as a trigger for prone positioning or neuromuscular blockade in ARDS.

Same formula as the app. Open it in the web app.

Related one-pagers

• ABG Interpretation in 6 StepsOxygenation: PaO₂, P/F ratio, A–a gradient.
• Ventilator Modes and SettingsACURASYS 2010 (n = 340, P/F <150) gave cisatracurium for 48 h and found lower adjusted 90-day mortality (hazard ratio 0.68). ROSE 2019 (n = 1,006, P/F <150 on PEEP ≥8) compared the same regimen with light sedation and stopped for futility: 90-day mortality 42.5% vs 42.8%. ATS 2024 gives a conditional recommendation for neuromuscular blockade in early severe ARDS, while ESICM 2023 recommends against routine continuous infusion, so the guidelines disagree. During paralysis, confirm deep sedation first, protect the eyes, keep the head of bed raised and remember that a paralyzed patient cannot show distress or seizure.
• ARDSPROSEVA (Guérin 2013; 466 patients) enrolled patients with P/F <150 on FiO₂ ≥0.6, PEEP ≥5 and Vt near 6 mL/kg PBW after a stabilization period, and proned them in sessions of at least 16 h. 28-day mortality was 16.0% vs 32.8% (hazard ratio 0.39, 95% CI 0.25–0.63) and 90-day mortality 23.6% vs 41.0% (HR 0.44). Complications did not differ except cardiac arrests, more frequent in the supine group. Earlier trials proned less severe patients for fewer hours and found no benefit.

Related calculators

• A-a Gradient
• Alveolar Gas Equation (PAO₂)
• Arterial Oxygen Content (CaO₂)
• Minute Ventilation
• Predicted Body Weight (PBW)
• SpO₂/FiO₂ Ratio
• Ventilator Rate for a Target PaCO₂

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your local protocols and medical direction. Not a substitute for clinical judgment.