PaO₂/FiO₂ (P/F) Ratio

Calculate the PaO₂/FiO₂ ratio to classify acute respiratory distress syndrome (ARDS) severity per Berlin definition.

PaO₂

FiO₂ (%)

FiO₂ expressed as percent (e.g., 50% = 0.50).

PEEP (cm H₂O)

PEEP ≥5 cm H₂O is required for ARDS classification.

Results

P/F ratio

160

Moderate ARDS (PEEP ≥5 cm H₂O).

FiO₂ fraction: 0.50 • PEEP: 10.0 cm H₂O

Berlin ARDS classification:

≥300: No ARDS
200–299: Mild ARDS
100–199: Moderate ARDS
<100: Severe ARDS

Assumes PEEP ≥5 cm H₂O and timing within 1 week of clinical insult.

How to Use This Calculator

Use arterial blood gas for PaO₂ and ventilator display for FiO₂ and PEEP at the time of sampling. Enter PaO₂ in mmHg or kPa; FiO₂ as percent. The calculator converts values automatically. Interpret the P/F ratio with PEEP and imaging findings to confirm Berlin criteria and guide management.

Formula

P/F ratio = PaO₂ (mmHg) ÷ FiO₂ (fraction).

Berlin definition of ARDS (JAMA. 2012).

Frequently Asked Questions

No. The Berlin definition uses PaO₂ from arterial blood gas. Pulse oximetry approximations (SpO₂/FiO₂) exist but differ. High altitude decreases PaO₂; consider local guidelines or base ratio on sea-level equivalents when available. Use stable ventilator settings and ABG drawn at steady state, typically 15–30 minutes after changes. Yes—measure after patient is in the target position to evaluate response. Berlin criteria require PEEP ≥5 cm H₂O. Increase PEEP if clinically appropriate before classifying ARDS.