P/F Ratio Calculator
Divide arterial oxygen pressure by the fraction of inspired oxygen and read the Berlin ARDS range.
What it does
The P/F ratio compares the oxygen pressure in arterial blood with the amount of oxygen being breathed. It is a compact way to describe hypoxemia that adjusts for the oxygen support a person is receiving, so a low PaO2 on high FiO2 is not read the same way as the same PaO2 on room air.
The arithmetic is one division. PaO2 comes from an arterial blood gas in mmHg, and FiO2 is the fraction of inspired oxygen written as a decimal, so 40% becomes 0.40. A PaO2 of 80 on 40% oxygen gives 80 / 0.40 = 200 mmHg. Room air is 21%, not 100%, which is a common source of error.
The Berlin definition, published in 2012, uses the ratio to grade acute respiratory distress syndrome. Mild is 200 to 300 mmHg, moderate is 100 to 200, and severe is 100 or less. The definition also requires a PEEP of at least 5 cm H2O, bilateral opacities on imaging, and a cause that is not fully explained by heart failure or fluid overload.
The number is only as good as the sample and the timing. Arterial blood gases are usually drawn after a change in support has had time to settle, and the ratio can shift with position, sedation, fluid balance and altitude. A single value near a boundary should be repeated rather than treated as a fixed category.
Use the ratio to describe severity and to follow a trend, not to name a disease. A low ratio in a patient with clear lungs, a normal heart and no risk factor is a reason to look for another explanation, not to diagnose ARDS by arithmetic.
How to use
- Enter the PaO2 from an arterial blood gas in mmHg.
- Enter the FiO2 as a percentage from 21 to 100. Do not enter the decimal fraction in this field.
- Enter the PEEP in cm H2O. The Berlin bands expect at least 5.
- Read the ratio and the range.
- Check that the sample was taken at a steady state after the current oxygen setting.
- Repeat the ratio after a change in support rather than comparing values taken at different FiO2 levels.
Formula
- PaO2
- Arterial oxygen partial pressure from a blood gas, in mmHg.
- FiO2
- Fraction of inspired oxygen, written as a decimal, so 40% is 0.40.
- P/F
- The ratio in mmHg; lower values mean worse oxygenation.
FiO2 must be converted from percent to a decimal before dividing. Entering 40 rather than 0.40 in a hand calculation divides by 40 instead of 0.40 and gives a ratio 100 times too small. The Berlin boundaries are 300, 200 and 100 mmHg; a ratio exactly on a boundary belongs to the more severe band on the low side, which is why 200 is moderate and 100 is severe in this tool. PEEP below 5 cm H2O means the ratio does not meet the Berlin definition even if the number falls in a band.
Worked example
Example: PaO2 80 on 40% oxygen with PEEP 5
Defaults: PaO2 80 mmHg, FiO2 40% and PEEP 5 cm H2O.
- Convert FiO2 to a decimal: 40% = 0.40.
- Divide PaO2 by the decimal: 80 / 0.40 = 200 mmHg.
- Compare with the Berlin boundaries: 200 is above 100 and at the top of the moderate band.
- Because the boundary belongs to the more severe side, 200 is called moderate rather than mild.
- The PEEP is 5 cm H2O, so it meets the support requirement in the definition.
- The tool shows 200 mmHg and the range Moderate range.
Reference table
| P/F ratio | Range | What it means |
|---|---|---|
| Above 300 mmHg | Above the ARDS cutoff | The ratio does not meet the Berlin oxygenation threshold |
| 201 to 300 mmHg | Mild | Mild range in the Berlin definition |
| 101 to 200 mmHg | Moderate | Moderate range; 200 itself is in this band |
| 100 mmHg or less | Severe | Severe range in the Berlin definition |
Limits
- Does not diagnose ARDS on its own; imaging, PEEP, fluid status and the clinical cause are also required.
- Depends on an arterial sample and a steady FiO2, so a value drawn during a change in support can mislead.
- Altitude, temperature correction and the blood gas analyzer can shift PaO2.
- A ratio near a boundary should be repeated rather than assigned a permanent category.
Common mistakes
- Entering FiO2 as 0.40 in a field that expects percent, or as 40 in a hand calculation that expects a decimal.
- Using a pulse oximeter saturation instead of PaO2 from an arterial blood gas.
- Forgetting that room air is 21%, not 0% or 100%.
- Reading a ratio below 300 as ARDS without checking PEEP, imaging and the heart failure alternative.
- Comparing ratios taken at different FiO2 levels or before the oxygen support had time to settle.
Try it yourself
Quick checks. Answers sit tucked under each question.
PaO2 60 mmHg on 50% oxygen. What is the P/F ratio?
50% is 0.50, so 60 / 0.50 = 120 mmHg, which is in the moderate range.
PaO2 50 mmHg on 80% oxygen. What is the P/F ratio and range?
80% is 0.80, so 50 / 0.80 = 62.5 mmHg, which the tool displays as 63 mmHg. That is 100 or less, so it is in the severe range.
PaO2 100 mmHg on 28% oxygen. What is the P/F ratio?
28% is 0.28, so 100 / 0.28 = 357.1 mmHg, which the tool displays as 357 mmHg. That is above the 300 mmHg ARDS cutoff.
Questions
What does the P/F ratio measure?
It measures how well oxygen moves from the air into arterial blood while accounting for how much oxygen is being breathed. A lower ratio means worse oxygenation.
Why is FiO2 entered as a percentage?
The form converts 40% to 0.40 for you. In a hand calculation, remember that 40% is 0.40, not 40.
What is a normal P/F ratio?
On room air, a healthy person often has a ratio above 400 mmHg. The Berlin definition starts at 300 mmHg or less, but a ratio alone does not define ARDS.
Does a P/F ratio of 200 mean moderate ARDS?
In the Berlin definition, 200 mmHg is the top of the moderate band, provided the other criteria are met. The number should be read with PEEP, imaging and the clinical cause.
Can I use SpO2 instead of PaO2?
No for this ratio. SpO2 is a saturation, not a pressure, and the P/F ratio is built from arterial PaO2. A different index, such as the ROX index, uses SpO2.
Does PEEP change the ratio?
PEEP can improve PaO2 and therefore raise the ratio, which is why the Berlin definition requires at least 5 cm H2O and why values taken at different PEEP levels are not directly comparable.
Sources
- ARDS Definition Task Force: the Berlin definition, JAMA, PubMed
- NHLBI: acute respiratory distress syndrome overview
- NHLBI ARDS Network: tools and protocols