PaFi Ratio Calculator (PaO₂/FiO₂)
Use this tool to calculate the PaFi ratio from arterial oxygen pressure (PaO₂) and inspired oxygen fraction (FiO₂).
For educational use only. Always interpret in full clinical context.
What is the PaFi ratio?
The PaFi ratio (also written P/F ratio) is the ratio of arterial oxygen partial pressure (PaO₂) to fraction of inspired oxygen (FiO₂). It is widely used in emergency medicine, critical care, and respiratory care to assess oxygenation efficiency.
In plain terms: it tells you how well oxygen is moving from the lungs into the blood, adjusted for how much oxygen the patient is receiving.
Formula used in this calculator
PaFi = PaO₂ / FiO₂
- PaO₂ is measured in mmHg from an arterial blood gas.
- FiO₂ must be entered as a fraction for the formula (for example, 40% = 0.40).
If you enter FiO₂ in percent, the calculator automatically converts it to fraction before computing the ratio.
How to use the PaFi calculator
Step-by-step
- Enter the measured PaO₂ value.
- Enter FiO₂ and choose whether it is percent or fraction.
- Optionally enter PEEP if you want an ARDS grading hint.
- Click Calculate PaFi.
The tool returns your PaFi ratio, an oxygenation interpretation band, and Berlin ARDS severity guidance (if PEEP is provided and at least 5 cmH₂O).
Interpreting PaFi values
Common oxygenation bands
- ≥ 400: Usually near-normal oxygenation.
- 300-399: Mild reduction in oxygenation efficiency.
- 200-299: Moderate impairment.
- 100-199: Severe impairment.
- < 100: Very severe impairment.
Berlin ARDS framework (requires full criteria)
When clinical ARDS criteria are met and PEEP/CPAP is at least 5 cmH₂O:
- Mild ARDS: PaFi 201-300
- Moderate ARDS: PaFi 101-200
- Severe ARDS: PaFi ≤ 100
Remember, PaFi alone does not diagnose ARDS. Imaging findings, timing, and edema origin are also required.
Example calculations
Example 1
PaO₂ = 90 mmHg, FiO₂ = 30% (0.30):
PaFi = 90 / 0.30 = 300
Example 2
PaO₂ = 75 mmHg, FiO₂ = 60% (0.60):
PaFi = 75 / 0.60 = 125
This indicates severe oxygenation impairment and would map to moderate ARDS severity if all ARDS criteria are satisfied.
Limitations and good practice
- PaFi can fluctuate with ventilator settings and timing of blood sampling.
- Incorrect FiO₂ assumptions can significantly distort the ratio.
- Hemodynamic state, shunt physiology, and lung mechanics all matter.
- Use trend data over time, not a single isolated value, whenever possible.
Best practice is to interpret PaFi alongside ABG trends, chest imaging, clinical exam, and ventilatory parameters.
Quick FAQ
Is FiO₂ entered as 50 or 0.50?
Either can work here. Just choose the correct unit in the calculator.
Can I diagnose ARDS from this alone?
No. This calculator supports oxygenation assessment, but ARDS diagnosis requires broader clinical criteria.
Why include PEEP?
Berlin ARDS severity grading assumes PEEP/CPAP of at least 5 cmH₂O.