# PaO₂/FiO₂ Ratio (P/F Ratio, ARDS Severity) Calculate the PaO₂/FiO₂ ratio and classify ARDS severity per the Berlin Definition (2012). Derived from ARDS Definition Task Force JAMA 2012, LITFL, and Rice 2007. Not medical advice. > Canonical page: https://elysiatools.com/en/tools/pao2-fio2-ratio - **Category:** Health - **Keywords:** pao2 fio2 ratio, pf ratio, ards, berlin definition, oxygenation index, acute respiratory distress syndrome, lung injury, hypoxemia, critical care, pulmonology, icu, ventilation ## Overview Calculate the PaO₂/FiO₂ ratio from arterial PaO₂ and inspired oxygen fraction, then view the corresponding ARDS severity category based on the Berlin Definition (2012). The result is provided as JSON and is not medical advice. ## Inputs - **PaO₂ (mmHg)** (number): Arterial partial pressure of oxygen (mmHg) from ABG. - **FiO₂ (inspired oxygen fraction)** (select) - **Decimal Places** (number) ## When to use - When reviewing arterial blood gas oxygenation in critical care or pulmonology workflows. - When comparing PaO₂ values across different FiO₂ settings. - When documenting an estimated P/F ratio and ARDS severity category from available clinical inputs. ## How it works - Enter PaO₂ in mmHg from an arterial blood gas result. - Select the FiO₂ value, from room air at 0.21 through 1.00 for 100% oxygen. - The tool calculates P/F using PaO₂ ÷ FiO₂. - The JSON result reports the ratio, ARDS category, mortality estimate when available, calculation method, and model reference. ## Use cases - Assess oxygenation severity from arterial blood gas and FiO₂ values. - Support ICU documentation and review of oxygenation trends. - Create a consistent P/F ratio calculation for teaching, research, or clinical reference. ## Frequently asked questions ### What is the PaO₂/FiO₂ ratio? It is PaO₂ divided by FiO₂ and is used to assess oxygenation impairment. ### Which units should I use for PaO₂? Enter arterial PaO₂ in mmHg. ### How should FiO₂ be entered? Use the inspired oxygen fraction as a decimal, such as 0.21 for room air, 0.30 for 30%, or 1.00 for 100% oxygen. ### How are ARDS categories classified? The categories are based on the P/F ratio: greater than 300, 200–300, 100–200, and 100 or below. ### Does this tool diagnose ARDS? No. It calculates and classifies the P/F ratio; ARDS diagnosis also requires clinical assessment and Berlin Definition criteria. ## Related tools - [A-a Gradient (Alveolar-Arterial O₂)](https://elysiatools.com/en/tools/a-a-gradient-calculator): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. Derived from StatPearls, LITFL, Cornell PICU, and Martin 1999. 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Five signs are each scored 0–2 at both time points: Appearance (skin color), Pulse (heart rate), Grimace (reflex irritability), Activity (muscle tone), and Respiration. Each time point totals 0–10. Interpretation: 7–10 reassuring; 4–6 moderately abnormal (may need assistance); 0–3 critically low (immediate resuscitation). The tool reports both the 1-min and 5-min scores, the change (delta), and per-time-point interpretation. A 5-min score, and especially the change from 1 to 5 min, indicates response to resuscitation; persistent low 5-min scores warrant continued resuscitation and reassessment at 10 min. Derived from Apgar 1953, reaffirmed by ACOG/AAP. The Apgar is a physiologic snapshot at a moment in time and is not, by itself, a marker of asphyxia or long-term neurologic outcome. 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