PaO₂/FiO₂ Ratio Calculator (P/F Ratio) with ARDS Severity, S/F Ratio and Altitude Correction
P/F ratio (PaO₂ ÷ FiO₂) in mmHg or kPa with Berlin and 2023 global ARDS oxygenation categories, the SpO₂/FiO₂ ratio and its cut-offs, the support needed for the criterion (ventilation, NIV/CPAP, high-flow oxygen), altitude-corrected P/F, the SOFA respiratory points and FiO₂ estimates for cannulas and masks.
How it works
P/F = PaO₂ (mmHg) ÷ FiO₂ (fraction) · Berlin / 2023 global, intubated with PEEP ≥ 5: mild 201–300, moderate 101–200, severe ≤ 100 · S/F = SpO₂ ÷ FiO₂ (SpO₂ ≤ 97%): mild 236–315, moderate 149–235, severe ≤ 148 · S/F ≈ 64 + 0.84 × P/F · altitude > 1,000 m: P/F × barometric pressure ÷ 760
Dividing the arterial oxygen tension by the inspired oxygen fraction normalises oxygenation for the amount of oxygen a patient is breathing: a PaO₂ of 80 mmHg is fine on room air (P/F 381) but alarming on 80% oxygen (P/F 100). The ratio anchors the 2012 Berlin definition of acute respiratory distress syndrome, which grades intubated patients with PEEP of at least 5 cmH₂O as mild (201–300), moderate (101–200) or severe (100 or less), and it is the respiratory part of the SOFA score. The 2023 global definition of ARDS (Matthay et al., published 2024) keeps the P/F of 300 or less, adds an SpO₂/FiO₂ ratio of 315 or less when SpO₂ is 97% or lower, and lets non-intubated patients qualify on high-flow nasal oxygen of at least 30 L/min or NIV/CPAP of at least 5 cmH₂O; severity is graded only for intubated patients. At altitude the Berlin definition multiplies P/F by barometric pressure ÷ 760 so patients in high cities are not over-classified. Oxygenation is only one ARDS criterion: onset within a week, bilateral opacities and oedema not explained by heart failure must also be present. FiO₂ on cannulas and masks is an estimate.
Worked example
PaO₂ 72 mmHg on 40% oxygen, intubated with 5 cmH₂O PEEP: P/F = 72 ÷ 0.40 = 180 mmHg (24.0 kPa), in the moderate ARDS range (101–200) if the other criteria are met; the Berlin cohort mortality for moderate ARDS was 32%. With SpO₂ 94%, S/F = 94 ÷ 0.40 = 235, also moderate (149–235), and the Rice regression estimates PaO₂ ≈ (235 − 64) ÷ 0.84 × 0.40 = 81 mmHg. The SOFA respiratory score is 3 points (P/F below 200 on ventilatory support).
Sources: A New Global Definition of Acute Respiratory Distress Syndrome (Matthay et al., Am J Respir Crit Care Med 2024), PubMed · Horowitz index (P/F ratio): normal values and Berlin categories with mortality (Wikipedia)
Frequently asked questions
What is a normal P/F ratio?
Healthy adults breathing room air have a P/F of about 400–500 mmHg (for example PaO₂ 95 ÷ 0.21 = 452). Below 300 indicates significant hypoxaemia; it is one of the criteria for ARDS.
What are the ARDS severity categories?
For intubated patients with PEEP of at least 5 cmH₂O: mild 201–300 mmHg, moderate 101–200 and severe 100 or less. The 2023 global definition adds S/F equivalents of 236–315, 149–235 and 148 or less when SpO₂ is 97% or lower.
Can I use SpO₂ instead of an arterial gas?
Yes, for screening and under the 2023 global definition: an SpO₂/FiO₂ ratio of 315 or less corresponds to a P/F of about 300, provided SpO₂ is 97% or lower. Above 97% the oximeter cannot distinguish a PaO₂ of 100 from 400.
How do I estimate FiO₂ on nasal cannula?
Roughly 21% plus 4% for each litre per minute, up to about 6 L/min (2 L ≈ 29%, 4 L ≈ 37%). Breathing pattern changes the real value, so P/F on low-flow oxygen is approximate; high-flow devices deliver the set FiO₂ more reliably.
Why correct the P/F ratio for altitude?
At altitude the barometric pressure and therefore PaO₂ are lower, so a healthy person in Mexico City (about 2,240 m, 580 mmHg) has a lower P/F than at sea level. The Berlin definition multiplies P/F by barometric pressure ÷ 760 above about 1,000 m.
What P/F ratio triggers prone positioning?
The PROSEVA trial proned patients with P/F below 150 mmHg on FiO₂ of at least 0.6 and PEEP of at least 5 for 16 hours a day or more, which reduced mortality. ECMO is commonly considered when P/F stays below about 80 despite optimised care.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside judgement, not a diagnosis. ARDS also requires acute onset, bilateral opacities not fully explained by effusion, collapse or nodules, and oedema not fully explained by heart failure or fluid overload.