What is the P/F ratio?

The P/F ratio is arterial oxygen tension (PaO₂) divided by the inspired oxygen fraction (FiO₂ expressed as a decimal). It summarises how efficiently the lungs are oxygenating blood, with lower values indicating worse gas exchange.

How does the Berlin definition classify ARDS?

With PEEP of at least 5 cmH₂O, a P/F ratio of 200 to 300 is mild ARDS, 100 to 200 is moderate, and below 100 is severe. A ratio above 300 does not meet the oxygenation criterion for ARDS.

Do I enter FiO₂ as a percent or a fraction?

Enter it as a percent, such as 60 for 60 percent oxygen. The tool converts it to the 0.60 fraction internally before dividing. Room air is 21 percent, which is the minimum the tool accepts.

My blood gas reports PaO₂ in kPa — does that matter?

Yes. The Berlin thresholds are defined in mmHg, so the tool converts kPa to mmHg using 1 kPa equals 7.5 mmHg before computing the ratio. Just select kPa as the unit and enter your value.

Is the P/F ratio enough to diagnose ARDS?

No. The Berlin definition also requires acute onset within a week, bilateral infiltrates on imaging not fully explained by cardiac failure or fluid overload, and a minimum PEEP. The ratio supplies only the oxygenation severity grade.

What is the P/F Ratio (PaO₂/FiO₂) Calculator — Berlin ARDS Criteria?

Divide PaO₂ by FiO₂ to produce the P/F ratio and classify ARDS severity as mild, moderate, or severe under the Berlin definition. A browser-based intensive care and respiratory physiology calculator that runs entirely on your device. It runs free in your browser on Gera Tools, with nothing uploaded.

P/F Ratio (PaO₂/FiO₂) Calculator — Berlin ARDS Criteria

Name: P/F Ratio (PaO₂/FiO₂) Calculator — Berlin ARDS Criteria
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The P/F ratio — arterial oxygen tension divided by inspired oxygen fraction — is the single most used number for grading hypoxaemic respiratory failure. This calculator computes it from a blood gas and ventilator setting, then classifies severity under the Berlin definition of acute respiratory distress syndrome, all in your browser.

How it works

The ratio divides PaO₂ (in mmHg) by FiO₂ expressed as a fraction:

P/F = PaO2_mmHg / (FiO2_percent / 100)

If your gas reports PaO₂ in kPa, the tool first converts it (1 kPa = 7.5 mmHg) because the Berlin thresholds are defined in mmHg. The Berlin definition, assuming PEEP of at least 5 cmH₂O, then grades the result: 200–300 is mild ARDS, 100–200 moderate, and below 100 severe; above 300 does not meet the oxygenation criterion.

Example and notes

A patient with a PaO₂ of 80 mmHg on 60% oxygen has a P/F ratio of 80 / 0.60 = 133, placing them in the moderate ARDS band. The ratio is sensitive to FiO₂, so a patient can move between categories simply by changing the oxygen setting — interpret it alongside PEEP and the ventilator strategy. The oxygenation grade is only one of the Berlin criteria; acute onset, bilateral infiltrates, and a non-cardiac cause must also be present to diagnose ARDS.

Berlin definition thresholds at a glance

The 2012 Berlin definition replaced the older AECC criteria and introduced PEEP as a mandatory context. All three severity categories require PEEP or CPAP of at least 5 cmH₂O:

ARDS severity	P/F ratio (mmHg)	Notes
No ARDS	Above 300	Does not meet oxygenation criterion
Mild	200 to 300	Must meet all other criteria plus PEEP
Moderate	100 to 200	Most common presentation at ICU admission
Severe	Below 100	Associated with higher mortality

Interpreting the ratio in clinical context

The P/F ratio is a snapshot, not a trajectory. A few practical points for clinical use:

FiO₂ dependence: The ratio changes with every adjustment to inspired oxygen. A patient titrated from 60% to 80% FiO₂ might appear to worsen (lower ratio) if the higher FiO₂ raised PaO₂ proportionally less than it changed the denominator. Standardise the FiO₂ when comparing serial values.

PEEP effect: Increasing PEEP recruits collapsed alveoli, which raises PaO₂ and can move a patient from the moderate to mild category even without any change in their underlying lung disease. The Berlin definition requires at least 5 cmH₂O PEEP to exclude spuriously good ratios on low or no PEEP.

kPa users: Many European and UK blood-gas analysers report in kPa. The conversion is 1 kPa = 7.5 mmHg. A PaO₂ of 10.7 kPa, for example, equals 80 mmHg — use the unit selector in this tool rather than converting manually.

Trend matters more than a single value: An improving P/F ratio over 12 to 24 hours in a ventilated patient generally suggests a response to treatment. A falling ratio despite optimised PEEP and FiO₂ is a concerning sign regardless of the absolute classification.

This tool is a calculation aid. Clinical decisions require the full patient assessment including imaging, clinical history, and the judgement of a qualified clinician.