The ratio shows how well the lungs transfer oxygen relative to how much is being given.
How to use the P/F Ratio Calculator
- PaO₂
- FiO₂
Results update as you type, and every field accepts the units shown in its menu.
How it works
Divide PaO₂ in mmHg by FiO₂ expressed as a decimal, such as 0.4 for 40%.
Inputs explained
| Field | What to enter |
|---|---|
| PaO₂ | In mmHg |
| FiO₂ | In % |
Example values
How the main result changes when PaO₂ changes and everything else stays at the example values above:
| PaO₂ (mmHg) | P/F ratio (mmHg) |
|---|---|
| 45 | 113 |
| 67.5 | 169 |
| 90 | 225 |
| 112.5 | 281 |
| 135 | 338 |
| 180 | 450 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
P/F = PaO₂ ÷ FiO₂ (as a decimal)Worked example
Inputs — PaO₂: 90 mmHg; FiO₂: 40 %.
Results — P/F ratio = 225 mmHg.
Frequently asked questions
What is normal?
Around 400–500 mmHg breathing room air.
Is a low P/F alone enough to diagnose ARDS?
No, timing, imaging and cause also count.
What formula does the P/F Ratio Calculator use?
P/F = PaO₂ ÷ FiO₂ (as a decimal).