P/F Ratio: A Fraction, a Pressure and the Missing Clinical Context

A worked P/F ratio example explains the difference between fraction-based arithmetic and the calculator's oxygen-percentage input.

Quick Medical Calculator · Reading label · 3 min read

Retrospective study sequence, not publication date

Editorial record

This source-linked educational article has been updated through AI-assisted evidence checks and editorial review. Verify information against current guidance and local protocols before applying it to patient care.

Citation methodology

Retrospective educational series: 17 September to 16 October 2026. This coverage window is not the publication date.

A P/F ratio becomes misleading quickly if inspired oxygen is entered as a percentage rather than a fraction. Even with the arithmetic correct, the ratio describes oxygenation under the recorded conditions. It does not independently establish acute respiratory distress syndrome, or ARDS.

Make the Units Visible

Use PaO2 in mmHg divided by FiO2 as a fraction. In a fictional example, PaO2 is 80 mmHg while FiO2 is 0.40:

P/F = 80 / 0.40 = 200 mmHg.

Entering 40 directly as the denominator gives 2, a hundredfold error. Conversely, putting 0.40 into an input explicitly expecting percent is also wrong. The local P/F calculator expects 40 in its inspired-oxygen percentage field and converts that to 0.40 internally. Enter PaO2 80 and oxygen 40 there to reproduce the ratio of 200.

Some blood-gas reports use kPa. A PaO2 of 10.6667 kPa is approximately 80 mmHg using 7.50062 mmHg per kPa. Dividing the kPa number by 0.40 gives 26.67 kPa, not a result directly comparable with mmHg-based bands. Convert or retain the units consistently; never compare the bare numbers as though they were the same scale.

The Support Conditions Belong With the Result

Keep the arterial specimen time, oxygen device, recorded FiO2 and respiratory support information together. A ratio calculated from one time point should not be matched to oxygen settings recorded after they changed. If inspired oxygen is uncertain, particularly with a variable-delivery device, the ratio has corresponding uncertainty.

For an arithmetic comparison only, the same PaO2 of 80 with FiO2 0.60 gives 133.3 mmHg, rather than 200. That does not tell you how the lungs actually responded to changing oxygen; it shows why the oxygen input cannot be omitted from the record.

Do Not Convert a Band Into a Diagnosis

The 2024 global ARDS definition combines oxygenation with other clinical criteria and explicitly broadens settings compared with the Berlin definition. Its categories incorporate respiratory-support and setting-specific considerations, as well as timing, imaging and assessment of the cause of pulmonary oedema. The ratio alone is insufficient. Global definition.

The local widget's oxygenation bands are Berlin-based; it does not implement the entire global diagnostic definition or assess every eligible support setting. An accompanying educational article cannot supply missing diagnostic observations or silently upgrade those widget labels.

Checkpoint: Does P/F 200 prove moderate ARDS? No. It can fall within an oxygenation band, but the remaining clinical and support criteria must be assessed under the named definition. A calculator result also supplies no ventilator setting, proning indication or rescue-treatment decision. Deteriorating oxygenation requires clinical assessment without waiting for a score.

Reading label: 27 September 2026. Day 11 of the retrospective educational series. This is a reading-sequence date, not a publication or clinical-review date.

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