What is the P/F ratio?

P/F ratio = PaO2 / FiO2. It's a simple index of oxygenation efficiency. Normal is >400. It's used to classify severity of hypoxemic respiratory failure and ARDS.

How is P/F ratio used in ARDS?

Berlin Definition: Mild ARDS: P/F 200-300. Moderate ARDS: P/F 100-200. Severe ARDS: P/F <100. Must have bilateral infiltrates, not fully explained by cardiac failure, with PEEP ≥5 cmH2O.

How do I calculate FiO2?

Room air = 21% (0.21). Nasal cannula: roughly 4% per L/min (1L=24%, 2L=28%, etc). Non-rebreather: 60-80%. High-flow: set on device. Ventilator: exact FiO2 displayed.

What are limitations of P/F ratio?

Doesn't account for CO2 or pH. Affected by PEEP, patient positioning, and shunt. May overestimate hypoxemia in hyperventilating patients. Use with clinical context.

What's normal P/F on room air?

Normal: PaO2 ~95-100 mmHg on FiO2 0.21 = P/F ~450-475. Values >400 are normal. Mild hypoxemia 300-400, moderate <300, severe <200.

Does PEEP affect P/F ratio?

Yes. Higher PEEP can improve P/F by recruiting alveoli. Berlin criteria require PEEP ≥5 cmH2O. Some use P/F on standardized PEEP for serial comparisons.

P/F Ratio Calculator

Q: How do I calculate FiO2?

Room air = 21% (0.21). Nasal cannula: roughly 4% per L/min (1L=24%, 2L=28%, etc). Non-rebreather: 60-80%. High-flow: set on device. Ventilator: exact FiO2 displayed.

Q: What are limitations of P/F ratio?

Doesn't account for CO2 or pH. Affected by PEEP, patient positioning, and shunt. May overestimate hypoxemia in hyperventilating patients. Use with clinical context.

Q: What's normal P/F on room air?

Normal: PaO2 ~95-100 mmHg on FiO2 0.21 = P/F ~450-475. Values >400 are normal. Mild hypoxemia 300-400, moderate <300, severe <200.

Q: Does PEEP affect P/F ratio?

Yes. Higher PEEP can improve P/F by recruiting alveoli. Berlin criteria require PEEP ≥5 cmH2O. Some use P/F on standardized PEEP for serial comparisons.

Calculate PaO2/FiO2 ratio for oxygenation assessment and ARDS classification.

Oxygenation Parameters

PaO2

mmHg

Partial pressure of arterial oxygen from ABG. Normal: 80-100 mmHg on room air.

FiO2

Fraction of inspired O2 as percentage. Room air = 21%, Nasal cannula ~24-44%, Face mask 35-60%.

Quick FiO2 Reference:

• Room air: 21%
• NC 1-6L: 24-44%
• Simple mask: 35-50%
• Non-rebreather: 60-80%

P/F Ratio

0

ARDS Berlin Criteria

P/F > 400Normal

P/F 300-400Mild Hypoxemia

P/F 200-300Mild ARDS

P/F 100-200Moderate ARDS

P/F < 100Severe ARDS

ARDS Diagnosis Requires: Bilateral infiltrates on CXR, not fully explained by cardiac failure, with PEEP ≥5 cmH2O.

Formula

P/F Ratio = PaO2 / FiO2

= 80 / 0.40 = 0

Frequently Asked Questions

How to Use

About the P/F Ratio Calculator

What it calculates

This p/f ratio calculator applies a validated clinical scoring method used in healthcare settings. PaO2/FiO2 for ARDS classification.

When to use it

Use as a clinical decision support aid alongside professional judgment — not as a standalone diagnosis.

Example

Adjust the inputs above to model your specific scenario with the P/F Ratio Calculator.

Explore More

P/F Ratio Calculator

Oxygenation Parameters

0

ARDS Berlin Criteria

Formula

About the P/F Ratio Calculator

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