PF Ratio Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

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Respiratory numbers often become useful only after the right relationship is calculated. PF Ratio Calculator focuses on the pulmonary measurements shown in this tool and turns them into the specific ratio, score, or estimate its engine supports.

What this calculator does

PF Ratio Calculator uses PaO₂ and FiO₂ (%) to produce the pao₂/fio₂ ratio used by this calculator. It only uses the fields shown in this calculator; it does not silently add tests, diagnoses, symptoms, or clinical variables that are absent from the interface. Uses standard health-calculator relationships where a stable equation exists; complex or policy-dependent tools expose assumptions/points so the result can be checked against the current clinical or public-health source.

How to use it

Enter or select PaO₂ and FiO₂ (%). Use measurements from the same respiratory assessment and confirm whether oxygen values are percentages, fractions, or pressures. If an input is unknown, it is better to leave the calculation unresolved than to invent a value.

How the calculation works

The PF ratio is PaO2 divided by FiO2 expressed as a decimal. For example, FiO2 40% is entered as 0.40 in the denominator, so PaO2 90 mmHg gives a PF ratio of 225 mmHg.

Example

Using the calculator’s prefilled demonstration values, the primary output is 225 mmHg, labeled “PaO₂/FiO₂ ratio”; PaO₂ is 90 mmHg and FiO₂ is 40%. That sample is useful for checking the calculation path, but real interpretation should use verified measurements from the same clinical or study context.

How to interpret the result

Lower PF ratios indicate worse oxygenation efficiency. ARDS classification also requires the relevant clinical/imaging criteria and specified respiratory support/PEEP context, so the ratio alone does not diagnose ARDS.

Limitations and notes

Respiratory measurements depend on technique, calibration, oxygen/ventilator settings, altitude, timing, and the patient population in which a rule was validated. A single calculated value should be interpreted with symptoms, examination, serial measurements, imaging, and the clinical question that prompted the test.