P/F Ratio Calculator (PaO2/FiO2)

How to calculate the P/F ratio

The P/F ratio (also the PaO2/FiO2 ratio; Spanish often says PaFi) divides arterial oxygen tension by the inspired oxygen fraction. It compares oxygenation when the patient is not on room air. This page asks for FiO2 as a percentage, as it usually appears on a ventilator or a Venturi mask.

The two forms are equivalent. With the on-page example, PaO2 80 mmHg and FiO2 50%: 80 ÷ (50 / 100) = 160. If you treated 50 as a fraction (0.50 written wrong), the result would be 1.6: a hundredfold error.

The ratio keeps the PaO2 unit (mmHg) because FiO2 is dimensionless. Reporting “P/F 160” without FiO2, PEEP, and the time of the sample loses clinical context.

Which values to use

Use figures taken at the same time. If you changed the oxygen minutes before the blood gas, the P/F ratio does not describe the current support.

• PaO2: from an arterial blood gas, in mmHg. It is not pulse-oximeter saturation (SpO2) and not a venous sample.
• FiO2: the fraction the device was delivering at that moment. Room air is 21% (0.21). Do not turn low-flow nasal-cannula litres per minute into a fixed percentage: that rule is inaccurate.
• If the analyser reports kPa: Berlin cut-offs are in mmHg. Multiply by 7.5 (1 kPa ≈ 7.5 mmHg) before calculating. This tool does not convert units.

A P/F ratio does not include haemoglobin, cardiac output, or PaCO2. Anaemia can leave an acceptable P/F with a low oxygen content.

Berlin values (with PEEP ≥ 5)

The Berlin definition (JAMA, 2012) grades ARDS hypoxemia with PEEP or CPAP of at least 5 cmH2O. This calculator applies those cut-offs to the ratio; it does not check the rest of the definition.

• P/F > 300: does not meet the Berlin oxygenation criterion. Respiratory disease may still be present.
• Mild: 200 < P/F ≤ 300. Observed mortality in the Berlin cohort: 27%.
• Moderate: 100 < P/F ≤ 200. Observed mortality: 32%. A value of exactly 200 belongs here.
• Severe: P/F ≤ 100. Observed mortality: 45%. A value of exactly 100 belongs here.

Those mortality figures describe the 2012 cohort; they do not predict one patient’s outcome. The asterisk on the result is a reminder about PEEP: without PEEP or CPAP ≥ 5 cmH2O, Berlin does not label ARDS severity.

What this number does not diagnose

A P/F ≤ 300 does not establish ARDS. Berlin also requires:

• Timing: onset within one week of a known insult or of new or worsening respiratory symptoms.
• Imaging: bilateral opacities on chest radiograph or CT not fully explained by effusion, collapse, or nodules.
• Origin of edema: respiratory failure not fully explained by cardiac failure or fluid overload.

This page does not read the radiograph, does not advise proning or ventilator changes, and does not replace the judgement of the team caring for the patient. Severe breathing difficulty is an emergency: do not wait on a number from a website.

Frequently asked questions

Are PaFi, P/F, and PaO2/FiO2 the same?

Yes. It is the same ratio. Spanish often says PaFi; the literature says P/F ratio.

Why does the formula multiply by 100?

Because FiO2 is entered here as a percentage (50, not 0.50). 80 ÷ (50 / 100) is the same as 80 ÷ 0.50. In Berlin, FiO2 is a decimal fraction.

Can I use pulse-oximeter saturation?

Not in this formula. SpO2/FiO2 is a different marker and does not replace arterial PaO2.

Does a P/F of 150 confirm moderate ARDS?

Not by itself. 150 falls in the moderate oxygenation band, but Berlin also requires PEEP or CPAP ≥ 5 cmH2O, timing, bilateral imaging, and the origin of edema.

What is PEEP here?

Positive end-expiratory pressure (or CPAP). Berlin does not grade ARDS from a P/F ratio alone if that pressure is below 5 cmH2O, because PEEP changes oxygenation.

Sources and references

Cut-offs, PEEP, and observed mortality were checked against the Berlin definition and professional reference material:

1. The ARDS Definition Task Force. Acute Respiratory Distress Syndrome: The Berlin Definition. JAMA. 2012;307(23):2526-2533. jamanetwork.com.
2. MSD Manual, Professional Version: Berlin definition of ARDS table.