About the P/F Ratio Calculator
The P/F ratio — arterial oxygen partial pressure (PaO₂) divided by the fraction of inspired oxygen (FiO₂) — is the standard bedside measure of how well the lungs transfer oxygen. Because it accounts for how much oxygen the patient is breathing, it allows a fair comparison between, say, a PaO₂ of 90 mmHg on room air and the same PaO₂ on 50% oxygen. A healthy person breathing room air has a ratio of roughly 400–500 mmHg.
Enter PaO₂ from an arterial blood gas (in mmHg or kPa) and the FiO₂ as a percentage. The calculator gives the ratio in both units and classifies it with the Berlin Definition of acute respiratory distress syndrome (ARDS): mild above 200 and up to 300 mmHg, moderate above 100 and up to 200, and severe 100 or less — each requiring PEEP or CPAP of at least 5 cmH₂O. At altitudes above about 1,000 m, enter the barometric pressure to apply the Berlin correction factor (PB ÷ 760).
A low P/F ratio does not by itself mean ARDS. The Berlin Definition also requires onset within a week of a known insult, bilateral opacities on imaging, and respiratory failure not fully explained by heart failure or fluid overload. The ratio is also used in SOFA scoring and for tracking oxygenation over time. This calculator is a clinical reference for adult patients.
With the default inputs, the p/f ratio is 180 mmHg. Change any value above to recalculate instantly.
How to use the p/f ratio calculator
- 1Choose the unit your blood gas reports PaO₂ in.
- 2Enter PaO₂ and the FiO₂ the patient was receiving at the time.
- 3Confirm whether PEEP or CPAP of at least 5 cmH₂O was applied.
- 4At high altitude, enter the local barometric pressure.
- 5Read the ratio and the Berlin oxygenation category.
Formula and method
FiO₂ is converted from a percentage to a fraction (50% = 0.50) before dividing. A PaO₂ entered in kPa is converted to mmHg by multiplying by 7.5006, and the ratio is reported in both units (the kPa thresholds are about 40, 26.7 and 13.3).
The Berlin Definition (ARDS Definition Task Force, 2012) grades hypoxaemia as mild (200 < P/F ≤ 300), moderate (100 < P/F ≤ 200) or severe (P/F ≤ 100), measured with PEEP or CPAP of at least 5 cmH₂O. In the validation cohort these stages carried mortality of about 27%, 32% and 45%. At altitudes above 1,000 m the ratio is multiplied by barometric pressure ÷ 760.
- PaO₂
- Arterial partial pressure of oxygen (mmHg)
- FiO₂
- Fraction of inspired oxygen (0.21–1.0)
- PB
- Barometric pressure (mmHg)
Worked examples
Moderate hypoxaemia on 50% oxygen
PaO₂ 90 mmHg ÷ 0.50 = 180 mmHg (24 kPa). With PEEP ≥ 5 this falls in the Berlin moderate band (100–200).
Severe ARDS range
PaO₂ 60 ÷ 0.80 = 75 mmHg, at or below 100, the Berlin threshold for severe ARDS.
Blood gas in kPa
12 kPa is about 90 mmHg; 90 ÷ 0.40 ≈ 225 mmHg, or 12 ÷ 0.40 = 30 kPa. That is in the mild band.
Altitude correction
The raw ratio is 70 ÷ 0.30 ≈ 233 mmHg. At a barometric pressure of 630 mmHg the Berlin correction gives 233 × 630/760 ≈ 193 mmHg, which is moderate rather than mild.
Frequently asked questions
What is a normal P/F ratio?+
A healthy adult breathing room air has a PaO₂ of about 80–100 mmHg, giving a P/F ratio of roughly 400–500 mmHg. The ratio falls with age and with any condition that impairs gas exchange.
What P/F ratio indicates ARDS?+
Under the Berlin Definition, ARDS requires a P/F ratio of 300 mmHg or less on PEEP or CPAP of at least 5 cmH₂O: mild above 200, moderate above 100 up to 200, and severe 100 or less — alongside the timing, imaging and origin criteria.
How do I convert FiO₂ for the calculation?+
Use FiO₂ as a fraction: room air is 0.21, 40% oxygen is 0.40. This calculator takes a percentage and converts it for you. Low-flow nasal cannula FiO₂ is only an estimate, which makes the ratio less reliable.
What is the P/F ratio in kPa?+
Divide the mmHg value by 7.5. The Berlin cut-offs of 300, 200 and 100 mmHg correspond to about 40, 26.7 and 13.3 kPa, which are often used in the UK and Europe.
Did the 2023 Global Definition of ARDS change the P/F cut-offs?+
No. The 2023 Global Definition keeps the same 300, 200 and 100 mmHg bands for intubated patients on PEEP ≥ 5 cmH₂O. It adds non-intubated ARDS (P/F ≤ 300 on high-flow nasal oxygen ≥ 30 L/min or NIV/CPAP ≥ 5 cmH₂O) and allows SpO₂/FiO₂ ≤ 315 when SpO₂ is 97% or less.
Is the P/F ratio the same as the Horowitz index?+
Yes. The PaO₂/FiO₂ ratio is also called the Horowitz index or oxygenation ratio. It is used in ARDS definitions, in the respiratory part of the SOFA score and to follow oxygenation trends.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.