What this calculator does
The PF ratio, short for PaO₂/FiO₂ ratio, is a simple division that compares how much oxygen is actually in the blood against how much oxygen the person is being given to breathe. It is one of the most widely used bedside measures of how well the lungs are transferring oxygen into the bloodstream.
A healthy person breathing room air, where FiO₂ is 21%, has a PaO₂ high enough to put the ratio well above 400. As lung function worsens, PaO₂ falls relative to the oxygen being delivered, and the ratio drops. The published Berlin definition uses fixed bands of this ratio as part of classifying the severity of ARDS (acute respiratory distress syndrome), which is why this calculator shows those bands alongside the number, for information only.
The formula
Divide the arterial oxygen partial pressure, PaO₂, by the fraction of inspired oxygen, FiO₂, expressed as a decimal rather than a percentage. Entering FiO₂ as a percentage here (for example 40 for 40%) is more familiar than working in decimals, so the calculator divides by 100 automatically before applying the formula.
| Term | Meaning |
|---|---|
| PaO₂ | Arterial oxygen partial pressure, measured in mmHg from an arterial blood gas sample. |
| FiO₂ | Fraction of inspired oxygen, the concentration of oxygen the person is breathing, from 21% on room air up to 100% on full supplemental oxygen. |
| PF ratio | PaO₂ divided by FiO₂ (as a decimal), used as one input into oxygenation severity scoring. |
| Berlin criteria | The 2012 consensus definition of ARDS, which uses PF ratio bands alongside timing, chest imaging and exclusion of cardiac failure as part of a broader diagnosis, not the ratio alone. |
The inputs explained
| Field | What to enter |
|---|---|
| PaO₂ (arterial oxygen partial pressure) (mmHg) | The arterial oxygen partial pressure from a blood gas result, in mmHg. |
| FiO₂ (fraction of inspired oxygen) (%) | The fraction of inspired oxygen as a percentage, for example 21 for room air or 100 for full supplemental oxygen. |
When to use it
Tracking oxygenation over a shift
Recalculating the PF ratio each time a blood gas is drawn gives a consistent number to compare against earlier readings, independent of whichever oxygen delivery device or flow rate was in use at the time.
Contributing to an ARDS severity assessment
The Berlin definition uses PF ratio bands as one of several criteria, alongside the timing of onset, chest imaging findings and ruling out heart failure as the cause; this calculator only produces the ratio and its band, not a diagnosis.
Comparing readings taken on different oxygen settings
Because the ratio already accounts for FiO₂, it allows a fairer comparison between two readings taken on different oxygen flow rates than comparing PaO₂ values alone would.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the PF ratio changes with PaO2 at a fixed FiO2
A fixed 40% FiO₂, across a range of PaO₂ readings.
| PaO₂ | PF ratio (PaO₂ ÷ FiO₂) | Oxygenation band (Berlin criteria, informational only) |
|---|---|---|
| 60 mmHg | 150 | Moderately reduced (Berlin "moderate" ARDS range, 101 to 200) |
| 80 mmHg | 200 | Moderately reduced (Berlin "moderate" ARDS range, 101 to 200) |
| 100 mmHg | 250 | Mildly reduced (Berlin "mild" ARDS range, 201 to 300) |
| 150 mmHg | 375 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 200 mmHg | 500 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 300 mmHg | 750 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 400 mmHg | 1,000 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
How the PF ratio changes with FiO2 at a fixed PaO2
A fixed 200 mmHg PaO₂, across a range of FiO₂ settings.
| FiO₂ | PF ratio (PaO₂ ÷ FiO₂) | Oxygenation band (Berlin criteria, informational only) |
|---|---|---|
| 21% | 952 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 30% | 667 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 40% | 500 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 50% | 400 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 60% | 333 | Above 300 - does not meet the Berlin ARDS oxygenation criterion on this measure alone |
| 80% | 250 | Mildly reduced (Berlin "mild" ARDS range, 201 to 300) |
| 100% | 200 | Moderately reduced (Berlin "moderate" ARDS range, 101 to 200) |
Questions
Does a PF ratio by itself diagnose ARDS?
No. The PF ratio is one component of the published Berlin definition, which also requires a specific timing of onset, bilateral opacities on chest imaging and exclusion of heart failure or fluid overload as the cause. This calculator only produces the ratio and its band, and any abnormal result should be discussed with a clinician rather than treated as a diagnosis on its own.
Why does the ratio use FiO2 as a decimal rather than a percentage?
The underlying formula is defined as PaO₂ divided by FiO₂ expressed as a fraction of 1, for example 0.4 rather than 40%. This calculator accepts FiO₂ as a percentage for convenience and converts it automatically.
What happens to the PF ratio if FiO2 is increased without any real change in lung function?
The ratio typically falls, because the same PaO₂ is now being achieved with more supplemental oxygen. This is a known limitation of the ratio, which is why FiO₂ and the clinical context are usually recorded alongside it rather than the ratio being read in isolation.
Is this the same as measuring oxygen saturation (SpO2)?
No. SpO₂ from a pulse oximeter estimates how saturated haemoglobin is with oxygen, while PaO₂ (used here) is the actual partial pressure of oxygen dissolved in arterial blood from a blood gas sample. The two are related but are not interchangeable, and the PF ratio specifically requires a PaO₂ value.
For another standard, fixed-criteria clinical scoring tool built the same cautious way, see the GCS calculator. For a related dialysis-adequacy measure, see the urea reduction ratio calculator.