P/F ratio calculator with FiO2 estimate (nasal cannula)
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Result
For exam practice only. This is a study aid, never a clinical decision: always follow the prescription, local protocols and the product information.
Calculate the PaO₂/FiO₂ ratio (P/F ratio), a measure of how well the lungs oxygenate the blood. Enter the PaO₂ from the arterial blood gas in mmHg or kPa and either the known FiO₂ or the nasal cannula flow, from which the FiO₂ is estimated with the 4% per litre rule of thumb. The result is given in both units with the Berlin definition bands.
Step-by-step solution
Estimate the FiO₂: about 4% per L/min on top of 21%
P/F ratio = PaO₂ ÷ FiO₂
Berlin definition bands
Key formulas
- P/F ratio (Berlin, PEEP ≥ 5)
- mild > 200–300, moderate > 100–200, severe ≤ 100 mmHg
- Nasal cannula
- FiO₂ ≈ 21% + 4% × L/min (1–6 L/min)
How to calculate the P/F ratio
- Find the FiO₂ as a fraction: room air is 0.21. On a nasal cannula, add about 4% per L/min: 2 L/min gives about 21 + 8 = 29%, or 0.29.
- Convert the PaO₂ to mmHg if needed: kPa × 7.5.
- Divide: P/F = PaO₂ ÷ FiO₂ = 80 ÷ 0.29 = 276 mmHg.
- Compare with the Berlin bands: above 200 up to 300 mild, above 100 up to 200 moderate, 100 or less severe (with PEEP of at least 5 cmH₂O).
Questions and answers
How accurate is the 4% rule for nasal cannulas?
It is a rough estimate for flows of 1 to 6 L/min. The real FiO₂ depends on how fast and deeply the patient breathes and on mouth breathing, so it is a teaching value, not a measurement.
What is a normal P/F ratio?
In healthy lungs breathing room air, about 400 to 500 mmHg (a PaO₂ of about 90–100 mmHg ÷ 0.21).
Where do the bands come from?
From the Berlin definition of ARDS (ARDS Definition Task Force, JAMA 2012): mild above 200 up to 300 mmHg, moderate above 100 up to 200 mmHg and severe 100 mmHg or less, with PEEP or CPAP of at least 5 cmH₂O for mild ARDS and PEEP of at least 5 cmH₂O for moderate and severe ARDS.
What about a simple face mask or non-rebreather?
Their FiO₂ varies even more with breathing. Exam tables often quote about 35–50% for a simple face mask at 5–10 L/min and 60–90% for a non-rebreather; a Venturi mask delivers a set FiO₂, which you can enter as a known value.
Practise it yourself
Practise Oxygen therapy & respiratory calculations
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