PF Ratio Calculator (PaO2/FiO2)
The PF ratio (PaO2/FiO2, also called the Horowitz or Carrico Index) is the standard bedside metric for assessing how well the lungs are transferring oxygen into the blood. Enter the arterial oxygen tension (PaO2) and the fraction of inspired oxygen (FiO2) to get the ratio, its ARDS severity band, and estimated mortality. Add the mean airway pressure to also compute the Oxygenation Index, a more ventilator-aware refinement used in critical care.
Formula
Worked example
A patient on 60% FiO2 (FiO2 = 0.60) has a PaO2 of 90 mmHg on ABG. P/F ratio = 90 / 0.60 = 150 mmHg, which falls in the moderate ARDS band (100-200 mmHg, ~32% mortality). If the ventilator shows a mean airway pressure of 14 cmH2O, the Oxygenation Index = (0.60 x 14 x 100) / 90 = 9.3, indicating moderate oxygenation failure.
What is the PF Ratio?
The PF ratio, formally the PaO2/FiO2 ratio, divides the partial pressure of oxygen in arterial blood (PaO2) by the fraction of oxygen in inspired air (FiO2). Also called the Horowitz Index or Carrico Index, it standardizes arterial oxygenation against the amount of supplemental oxygen the patient is receiving, making it possible to compare oxygenation across different levels of oxygen support. At sea level on room air (FiO2 = 0.21), a healthy adult typically has a PaO2 of 90-100 mmHg, giving a P/F ratio of approximately 430-480 mmHg. A value below 300 mmHg suggests impaired gas exchange, and a value below 200 mmHg meets one of the criteria for acute respiratory distress syndrome under the 2012 Berlin Definition.
How to use this calculator
Enter the PaO2 from the most recent arterial blood gas report. You can switch the PaO2 unit between mmHg and kPa using the toggle above the input. Then enter the FiO2 as a percentage: room air is 21%, a patient receiving 60% oxygen via ventilator would be 60. For ventilated patients, toggle on the Oxygenation Index option and add the mean airway pressure (MAP) from the ventilator display in cmH2O. The calculator immediately shows the P/F ratio, the ARDS severity band, the estimated hospital mortality for that band, and, if MAP is entered, the Oxygenation Index with its interpretation. Severity classification follows the 2012 Berlin Definition, which also requires PEEP of at least 5 cmH2O and acute onset within 1 week of a known clinical insult, so clinical context must always accompany the number.
ARDS Berlin Definition and why the P/F ratio matters
The 2012 Berlin Definition replaced the older American-European Consensus Conference definition and stratified ARDS into mild (P/F 200-300 mmHg), moderate (P/F 100-200 mmHg), and severe (P/F below 100 mmHg) categories on a minimum PEEP of 5 cmH2O. The classification predicts escalating hospital mortality: approximately 27% for mild, 32% for moderate, and 45% for severe. Because the P/F ratio is included in severity scores such as SOFA, APACHE IV, and SAPS II, it drives treatment intensity thresholds, including when to consider prone positioning (typically P/F below 150 mmHg), neuromuscular blockade, and ECMO referral. Trending the ratio over time after each ventilator change is at least as informative as any single reading.
Oxygenation Index and when to use it
The Oxygenation Index (OI) extends the P/F concept by including mean airway pressure: OI = (FiO2 x MAP x 100) / PaO2. Because MAP captures how hard the ventilator is working to maintain oxygenation, two patients with identical P/F ratios but different MAP values have meaningfully different lung mechanics. OI is especially useful in neonatal and pediatric critical care, where it is well-validated for ECMO referral thresholds (OI >= 25-40 in neonates). In adults, OI above 25 suggests severe oxygenation failure and above 40 raises concern for ECMO-level severity. The S/F ratio (SpO2/FiO2) can substitute when arterial blood gas is unavailable, with an S/F below 235 approximating a P/F below 200 mmHg.
ARDS Berlin Definition severity bands
| P/F Ratio (mmHg) | ARDS Severity | Estimated Hospital Mortality |
|---|---|---|
| > 400 | Normal (no ARDS) | < 10% |
| 300-400 | Below normal (no ARDS) | < 10% |
| 200-300 | Mild ARDS | ~27% |
| 100-200 | Moderate ARDS | ~32% |
| < 100 | Severe ARDS | ~45% |
Berlin Definition (2012). PEEP >= 5 cmH2O required. Mortality rates are approximate hospital figures from the original validation cohort.
Frequently asked questions
What is a normal PF ratio?
At sea level, a healthy adult breathing room air or supplemental oxygen typically has a P/F ratio of 400-500 mmHg. A value above 400 mmHg is generally considered normal. Values between 300 and 400 mmHg suggest mild impairment, and anything below 300 mmHg meets the oxygenation criterion for acute respiratory distress syndrome under the Berlin Definition.
How do I calculate the P/F ratio by hand?
Divide PaO2 (in mmHg) by FiO2 expressed as a decimal. For example, if PaO2 = 70 mmHg and the patient is on 40% oxygen (FiO2 = 0.40), then P/F ratio = 70 / 0.40 = 175 mmHg, which falls in the moderate ARDS range. If your blood gas report gives PaO2 in kPa, multiply by 7.5 to convert to mmHg first.
What is the difference between P/F ratio and Oxygenation Index?
The P/F ratio divides PaO2 by FiO2 and reflects how efficiently the lungs transfer oxygen. The Oxygenation Index adds mean airway pressure (MAP) to the formula: OI = (FiO2 x MAP x 100) / PaO2. Because it accounts for ventilator support intensity, OI is considered a more complete picture of lung function in mechanically ventilated patients. A higher OI means worse oxygenation relative to the support being provided.
Can I use SpO2 instead of PaO2?
Yes, the S/F ratio (SpO2 / FiO2) is a non-invasive surrogate that is useful when arterial blood gas sampling is not available. An S/F ratio below 235 approximates a P/F ratio below 200 mmHg (moderate ARDS), and an S/F below 315 approximates a P/F below 300 mmHg (ARDS threshold). The S/F ratio loses reliability at SpO2 values above 97% where the oxyhemoglobin dissociation curve is flat.
What FiO2 does room air correspond to?
Room air contains approximately 20.9% oxygen, conventionally rounded to 21% in clinical calculations. For nasal cannula delivery, each 1 L/min of oxygen flow adds roughly 4% to the FiO2: 1 L/min gives approximately 24%, 2 L/min gives 28%, and so on up to 6 L/min at approximately 44%. For devices with higher flow rates such as non-rebreather masks and high-flow nasal cannula, the FiO2 table in this calculator provides standard estimates.
Does PEEP affect the P/F ratio calculation?
PEEP itself does not enter the P/F ratio formula, but it is required for ARDS classification. The Berlin Definition specifies that the P/F ratio must be measured on a minimum PEEP (or CPAP) of 5 cmH2O to qualify as ARDS. Increasing PEEP often improves the P/F ratio by recruiting collapsed alveoli, so two readings at different PEEP levels are not directly comparable. The Oxygenation Index partially addresses this because mean airway pressure incorporates PEEP.