PaO₂ / FiO₂ Ratio Calculator
Respiratory failure severity · Berlin ARDS definition · ATS/ERS/ESICM 2024
PaO₂ (mmHg) ÷ FiO₂ (fraction) · Normal breathing room air: ≥ 400 mmHg
201–300 · Moderate 101–200 · Severe ≤ 100 — all require PEEP ≥ 5 cmH₂O
P/F ≈ (SpO₂ / FiO₂) × 5 (approximate, not a substitute for ABG)
| Severity | P/F ratio (mmHg) | Criteria | Hospital mortality | Key management |
|---|---|---|---|---|
| Normal | ≥ 301 | No ARDS criteria met | — | Treat underlying cause; reassess |
| Mild ARDS | 201 – 300 | PEEP ≥ 5 cmH₂O; bilateral opacities; acute onset < 1 week; non-cardiogenic | 27% | Low tidal volume (6 mL/kg PBW); PEEP optimisation; prone positioning consider |
| Moderate ARDS | 101 – 200 | As above + worsening oxygenation | 32% | Prone positioning ≥ 16 hr/day; consider NMB; lung-protective ventilation |
| Severe ARDS | ≤ 100 | As above + refractory hypoxia | 45% | Prone > 16 hr; NMB infusion; VV-ECMO if P/F < 80 despite optimisation; iNO consider |
P/F Ratio = PaO₂ (mmHg) ÷ FiO₂ (fraction) · Berlin ARDS definition 2012 · PEEP ≥ 5 cmH₂O required for ARDS staging · No patient data stored or transmitted
- 1.ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, et al. Acute respiratory distress syndrome: the Berlin definition. JAMA. 2012;307(23):2526–2533. doi:10.1001/jama.2012.5669 (Berlin ARDS definition — primary reference)
- 2.Thompson BT, Chambers RC, Liu KD. Acute respiratory distress syndrome. N Engl J Med. 2017;377(6):562–572. doi:10.1056/NEJMra1608077 (ARDS pathophysiology and management review)
- 3.Papazian L, Aubron C, Brochard L, et al. Formal guidelines: management of acute respiratory distress syndrome. Ann Intensive Care. 2019;9(1):69. doi:10.1186/s13613-019-0540-9
- 4.Guérin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). N Engl J Med. 2013;368(23):2159–2168. doi:10.1056/NEJMoa1214103 (Prone positioning landmark RCT)
- 5.Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for ALI and ARDS (ARDSNet). N Engl J Med. 2000;342(18):1301–1308. (6 mL/kg PBW lung-protective ventilation)
- 6.Combes A, Hajage D, Capellier G, et al. Extracorporeal membrane oxygenation for severe ARDS (EOLIA trial). N Engl J Med. 2018;378(21):1965–1975. doi:10.1056/NEJMoa1800385 (VV-ECMO in severe ARDS)
- 7.Writing Group for the Alveolar Recruitment for ARDS Trial (ART) Investigators. Effect of lung recruitment and titrated PEEP vs low PEEP on mortality in patients with ARDS. JAMA. 2017;318(14):1335–1345.
- 8.Fan E, Del Sorbo L, Goligher EC, et al. An official ATS/ERS/ESICM/SCCM/SRLF clinical practice guideline: mechanical ventilation in adult patients with ARDS (2017, updated 2024). Am J Respir Crit Care Med. 2017;195(9):1253–1263.
- 9.Villar J, Ferrando C, Martínez D, et al. Dexamethasone treatment for the acute respiratory distress syndrome (DEXA-ARDS). Lancet Respir Med. 2020;8(3):267–276. (Corticosteroids in ARDS)
For qualified clinical professionals only. This P/F Ratio Calculator is an educational decision-support tool. It is not a diagnostic instrument and does not replace arterial blood gas interpretation, chest imaging review, clinical examination, or multi-disciplinary critical care assessment.
The PaO₂/FiO₂ ratio is one component of the Berlin ARDS definition. ARDS diagnosis requires all four criteria: (1) acute onset within 1 week of clinical insult, (2) bilateral opacities on chest imaging not explained by effusions or collapse, (3) respiratory failure not fully explained by cardiac failure or fluid overload, and (4) P/F ratio below 300 mmHg with PEEP ≥ 5 cmH₂O. The P/F ratio alone is insufficient to diagnose ARDS.
FiO₂ estimation from non-invasive oxygen delivery devices (nasal cannula, venturi mask, non-rebreather) is inherently imprecise — actual delivered FiO₂ varies with tidal volume, respiratory rate, mask fit, and flow rate. The quick-select chips in this calculator represent typical estimates only. For mechanically ventilated patients, use the set FiO₂ from the ventilator display.
The SpO₂-based P/F estimate shown by this tool is a rough approximation [P/F ≈ SpO₂/FiO₂ × 5] derived from the ROX index literature. It is not equivalent to ABG-derived PaO₂ and should not be used for ARDS staging. Obtain ABG for definitive P/F ratio calculation.
Mortality estimates shown in the Berlin severity table are derived from the original 2012 Berlin validation cohort. Individual patient outcomes depend on aetiology, comorbidities, time to treatment, ventilatory management, and institution-specific care quality.
This tool does not store, transmit, or process patient-identifiable information. All calculations run locally in the user's browser. The authors assume no liability for clinical decisions based on this tool.
Last reviewed: January 2026 · Berlin ARDS definition 2012 · ATS/ERS/ESICM/SCCM 2024 mechanical ventilation guidelines · PROSEVA · ARDSNet · EOLIA incorporated
