PaO2/FiO2 Ratio Calculator

About this calculator

The PaO2/FiO2 ratio (P/F ratio) divides the partial pressure of oxygen in arterial blood (PaO2, from an arterial blood gas, in mmHg) by the fraction of inspired oxygen (FiO2, expressed as a decimal — 0.21 for room air, higher on supplemental oxygen or a ventilator). A healthy person breathing room air has a P/F ratio around 400-500; the ratio drops as the lungs become less able to transfer oxygen into the blood relative to what’s being delivered.

Under the Berlin criteria, the P/F ratio is the key measure used to grade the severity of ARDS (acute respiratory distress syndrome) in a mechanically ventilated patient with bilateral lung infiltrates: mild ARDS is a P/F ratio of 200-300, moderate is 100-200, and severe is below 100. Intensivists recalculate it repeatedly through a patient’s ICU stay to track whether oxygenation is improving or worsening and to guide decisions like ventilator settings and prone positioning.

This calculator divides your arterial oxygen level by the inspired oxygen fraction and grades the result using the Berlin ARDS severity criteria.