Calculates the PaO2/FiO2 ratio from a blood gas and the inspired oxygen, and places it in the Berlin ARDS severity categories.
Result
Enter PaO2 and FiO2.
Sources
ATS 2024: Conditional · 5 studies
ConditionalATS 2024 higher PEEP in moderate to severe ARDS.
5
studies
8,985
citations
3,607
PubMed papers
Evidence
Guideline recommendation
“We suggest the use of … higher PEEP without lung recruitment maneuvers as opposed to lower PEEP in patients with moderate to severe ARDS.”
American Thoracic Society, 2024
Conditional, low to moderate certainty
Am J Respir Crit Care Med · cited 488 times
PubMed · Free full text · DOI
Known limitations
In a patient-level analysis of 4,188 patients with ARDS, mortality was 27% in mild, 32% in moderate and 45% in severe ARDS, and the Berlin categories predicted mortality slightly better than the 1994 definition (area under the curve 0.577 against 0.536).
The 2024 global definition keeps the ratio cut-offs, adds patients on high-flow nasal oxygen at 30 L/min or more, accepts SpO2/FiO2 of 315 or less and lung ultrasound, and drops the PEEP requirement in resource-limited settings.
Am J Respir Crit Care Med · cited 916 times
PubMed · Free full text · DOI
In 466 patients with a ratio below 150 mm Hg on FiO2 of at least 0.6, prone positioning for at least 16 hours a day lowered 28-day mortality from 32.8% to 16.0%, and 90-day mortality from 41.0% to 23.6%.
N Engl J Med: Journal Metrics Score 29.8, Q1 · cited 4,121 times
In models fitted to data from 36 ventilated and 57 spontaneously breathing patients studied at four to eight FiO2 levels, changing the FiO2 alone moved 30% of patients into another oxygenation category.
Crit Care: Journal Metrics Score 7.4, Q1 · cited 178 times
PubMed · Free full text · DOI
The American-European Consensus Conference defined acute lung injury as a ratio of 300 or less and ARDS as 200 or less, regardless of PEEP, the definition used until 2012.
The original paper has been cited 8,985 times, and 3,607 PubMed articles mention the PaO2/FiO2 ratio in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 9, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
100 or less · 101 to 300 · Above 300
Severe range: senior critical care review. Mild or moderate range: confirm the diagnosis, then follow the guideline. Outside the ARDS range, not necessarily normal.
Next steps
Avoid common errors
7 common errors to avoid
and 5 more
When the ratio misleads
1FiO2 as a percentage
Divide by the fraction, not the percentage: 56 / 0.50 is 112, but 56 / 50 is 1.1. Enter FiO2 in the unit shown beside the box.
2Estimated FiO2
On nasal cannula or a simple face mask the delivered FiO2 is not known and varies with breathing, so the ratio is only approximate.
3FiO2 changes the ratio
The same lung gives different ratios at different FiO2; in one modeling study 30% of patients changed category with FiO2 alone (Karbing 2007). Compare like with like.
4PEEP is part of the definition
The Berlin categories assume PEEP of at least 5 cm H2O (CPAP counts only for mild ARDS). More PEEP usually raises the ratio, so report it with the result.
5Not a diagnosis on its own
Unilateral pneumonia, atelectasis, cardiogenic pulmonary edema and pulmonary embolism also lower the ratio. ARDS needs the timing, imaging and origin criteria too.
6Altitude
Above 1,000 m, both definitions correct the ratio: multiply it by barometric pressure ÷ 760 before applying the cut-offs.
7kPa and mm Hg
A ratio in kPa is about 7.5 times smaller: 300 mm Hg is 40 kPa. Check which unit a cut-off uses.
Formula
PaO2 in mm Hg divided by FiO2 as a fraction; Berlin categories at 300, 200 and 100.
PaO2 ÷ FiO2
PaO2 in mm Hg divided by FiO2 as a fraction; Berlin categories at 300, 200 and 100.
Formula and categories
The formula
P/F = PaO2 (mm Hg) ÷ FiO2 (0.21 to 1.0)
PaO2 in kPa is converted with 1 kPa = 7.50062 mm Hg. The result is rounded to a whole number and the category chosen on that number.
| Category | P/F, mm Hg | Mortality | Meaning |
|---|---|---|---|
| Severe ARDS range | 100 or less | 45% mortality | In the ARDS range for severe ARDS: 100 mm Hg or less (Berlin Definition). |
| Moderate ARDS range | 101 to 200 | 32% mortality | In the range for moderate ARDS: above 100 and up to 200 mm Hg (Berlin Definition). |
| Mild ARDS range | 201 to 300 | 27% mortality | In the range for mild ARDS: above 200 and up to 300 mm Hg (Berlin Definition). |
| Above the ARDS range | above 300 | – | Above 300 mm Hg, the ratio does not meet the oxygenation criterion for ARDS. It can still be well below normal, which in healthy adults breathing room air is about 380 to 480. |
Mortality by category from the Berlin Definition’s pooled analysis of 4,188 patients with ARDS. The categories assume PEEP of at least 5 cm H2O (CPAP counts only for mild ARDS).
Earlier definition (1994)
2024 global definition
Units
Test cases
3 published cases, each also a test case
3 published cases; the calculator reproduces every ratio.
Worked examples
A young man with polytrauma and bilateral lung contusions had severe hypoxemic respiratory failure. The first emergency department blood gas showed a PaO2 of 56 mm Hg on FiO2 0.50, and the report gives a PaO2/FiO2 of 112 mm Hg.
“PaO2 56 mmHg on FiO2 0.50 (PaO2/FiO2 112 mmHg)”
PEEP was not reported for this gas. After ICU admission the ratio was 78 mm Hg on FiO2 1.0 and PEEP 8 cm H2O, and the report classified the ARDS as severe.
Entered as: PaO2: 56 mm Hg; FiO2: 0.5 fraction.
Source: Huang Z et al., Patient Prefer Adherence 2026 (PubMed · Free full text · DOI)
A 78-year-old man developed severe pneumonia after aspirating river water in a bicycle accident. Readmitted to intensive care with hypoxemic respiratory failure, he had a PaO2 of 60 mm Hg on noninvasive ventilation with FiO2 55% and PEEP 8 cm H2O; the report gives a ratio of 109.
“a PaO2 of 60 mmHg, corresponding to a PaO2/FiO2 ratio of 109”
Bacteremic tularemia was diagnosed later; he needed venovenous ECMO. On noninvasive ventilation this is nonintubated ARDS under the 2024 definition, which does not grade severity.
Entered as: PaO2: 60 mm Hg; FiO2: 55 %.
Source: Wenglinski C et al., Front Med (Lausanne) 2026 (PubMed · Free full text · DOI)
A previously healthy 40-year-old man with disseminated tuberculosis was intubated for severe respiratory distress. On volume control with FiO2 1.0 and PEEP 10 cm H2O, the PaO2 was 139 mm Hg; the report gives a PaO2/FiO2 of 139 mm Hg, moderate ARDS by the Berlin criteria.
At FiO2 1.0 the ratio equals the PaO2.
Entered as: PaO2: 139 mm Hg; FiO2: 100 %.
Source: Zeizzoun R et al., Cureus 2026 (PubMed · Free full text · DOI)
FAQ
7 questions and answers
and 5 more answers
Questions
Divide the arterial PaO2 in mm Hg by the FiO2 written as a fraction. A PaO2 of 56 mm Hg on 50% oxygen (FiO2 0.50) gives 56 / 0.50 = 112 mm Hg. If the PaO2 is in kPa, multiply it by 7.5 first, or divide the ratio in kPa by 0.133.
A healthy young adult breathing room air at sea level has a PaO2 of about 80 to 100 mm Hg on an FiO2 of 0.21, a ratio of roughly 380 to 480 mm Hg. PaO2 falls with age and altitude, so healthy older adults can be in the mid-300s. The ARDS definitions start at 300 mm Hg or less.
The Berlin Definition grades ARDS by the ratio: mild above 200 up to 300 mm Hg, moderate above 100 up to 200, and severe 100 or less, measured with PEEP of at least 5 cm H2O (CPAP counts only for mild ARDS). The ratio is only one criterion: ARDS also needs onset within one week, bilateral opacities on imaging, and respiratory failure not fully explained by heart failure or fluid overload.
It keeps the Berlin ratio cut-offs and adds patients on high-flow nasal oxygen at 30 L/min or more, allows an SpO2/FiO2 of 315 or less (when SpO2 is 97% or below) in place of a blood gas, and accepts lung ultrasound for the imaging criterion. In resource-limited settings it does not require PEEP or a minimum oxygen flow.
Yes. The ratio changes with the FiO2 at which it is measured, even when the lung itself has not changed. In a modeling study using data from 93 patients, changing the FiO2 alone moved 30% of patients into another hypoxemia category (Karbing 2007). Compare ratios measured at similar FiO2 and PEEP.
In the PROSEVA trial, prone positioning for at least 16 hours a day was tested in severe ARDS defined as a ratio below 150 mm Hg on FiO2 of at least 0.6, PEEP of at least 5 cm H2O and a tidal volume near 6 mL/kg of predicted body weight. 28-day mortality was 16.0% with prone positioning against 32.8% supine.
Low-flow devices deliver a variable FiO2 that depends on the breathing pattern, so any figure is an estimate and the ratio is approximate. The ratio is most reliable on a ventilator, CPAP or high-flow device with a set FiO2.
Further reading
9 references with PubMed and DOI links
9
references
6
free full text
From ARDS Definition Task Force; Ranieri VM et al., JAMA 2012 to the newest case reports.
References
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 6, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
This calculator supports, and does not replace, clinical judgment. It is meant for healthcare professionals and is not medical advice for patients. Report an error through the contact page.
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Which published the study of how FiO2 changes the ratio, with its current metrics.
A leading critical care journal, with its current metrics.