How this calculator works
The P/F ratio divides arterial oxygen partial pressure by the inspired oxygen fraction. Because the fraction has no units, the result retains the pressure unit. Clinical definitions commonly use mmHg. A P/F ratio alone cannot diagnose ARDS: timing, imaging, origin of oedema, respiratory support and other criteria also matter. Inspired oxygen estimates from low-flow devices may be uncertain. The result is an estimate for information only and is not medical advice.
Formula
P/F = PaO2/FiO2. PaO2 = (P/F) x FiO2. FiO2 = PaO2/(P/F). Use 0.40, not 40, for 40% oxygen in the arithmetic.
Worked example
PaO2 80 mmHg on 40% oxygen gives P/F = 80/0.40 = 200 mmHg. A ratio of 300 mmHg on 30% oxygen implies PaO2 90 mmHg.
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Frequently asked questions
Can I enter kPa?
Yes. Pressures convert to the engine base unit, then the output selector can show mmHg or kPa. A threshold stated in mmHg must not be applied to a numerical kPa result.
Is oxygen saturation interchangeable with PaO2?
No. PaO2 is a blood gas partial pressure. Pulse oximeter saturation cannot be inserted into this formula.
Does this diagnose ARDS?
No. The ratio is one part of an oxygenation assessment and requires the other clinical definition criteria.