PaO2/FiO2 Ratio Calculator

How it is calculated

The P/F ratio is PaO2 ÷ FiO2, where PaO₂ is the arterial oxygen pressure in mmHg and FiO₂ is the inspired oxygen as a fraction. You enter FiO₂ as a percent (for example 40, or 21 for room air); the tool divides by 100 first and shows that fraction too. The ratio is rounded to a whole number. Grades follow the Berlin cut-offs: above 300 normal, above 200 up to 300 mild, above 100 up to 200 moderate, and 100 or less severe, so the boundary values 300, 200 and 100 fall in the worse grade. PaO₂ must be above 0 and at most 800 mmHg, and FiO₂ above 0 and at most 100 %; anything else gives no result. The tool grades any ratio that passes these limits; a clinical ARDS diagnosis also needs other criteria.

Common mistakes

• Typing FiO₂ as 0.4 instead of 40: the tool reads 0.4 % and prints a ratio of 18,750, graded Normal / no ARDS.
• Using oxygen saturation (94) instead of the arterial pressure PaO₂ in mmHg; the ratio only makes sense with the pressure.
• Entering PaO₂ in kPa without converting to mmHg (about 7.5 mmHg per kPa); the ratio then looks several times too low.
• Reading a grade as an ARDS diagnosis: the tool labels any ratio, even for a patient who is not ventilated or has no lung infiltrates.
• Entering a PaO₂ above 800 by a unit slip: it now gives no result, but a wrong value below that limit is still graded, so check the units yourself.