PaO₂/FiO₂ (P/F) Ratio Calculator

About this calculator

The PaO₂/FiO₂ ratio, often just called the 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 a patient is receiving (FiO₂, expressed as a decimal — 0.21 for room air, 1.0 for 100% oxygen). It is the core measurement used to judge how well the lungs are transferring oxygen into the blood relative to how much oxygen is being delivered.

This ratio is the backbone of the Berlin Definition of ARDS (Acute Respiratory Distress Syndrome), which grades severity as mild (200-300 mmHg), moderate (100-200 mmHg), or severe (less than 100 mmHg), each requiring a minimum PEEP or CPAP of 5 cmH₂O. Intensivists, pulmonologists, and respiratory therapists in the ICU use it to classify ARDS severity, decide whether to escalate to prone positioning, higher PEEP, or ECMO referral, and track a patient's oxygenation trend over the course of mechanical ventilation.

This calculator takes the PaO₂ from a blood gas and the current FiO₂ setting and returns the P/F ratio instantly for bedside decision-making.