Alveolar Gas Equation Calculator

About this calculator

The alveolar gas equation, PAO₂ = FiO₂ × (Patm − PH₂O) − PaCO₂/RQ, calculates the partial pressure of oxygen inside the alveoli — the tiny air sacs where gas exchange with the blood actually happens — from the fraction of inspired oxygen (FiO₂), barometric pressure, water vapor pressure (a constant 47 mmHg at body temperature), arterial CO₂ (PaCO₂) and the respiratory quotient (RQ, typically 0.8 on a mixed diet).

Clinicians use this equation specifically to calculate the A-a gradient — the difference between calculated alveolar oxygen and the patient’s measured arterial oxygen (PaO₂) — because a widened gradient points to a problem with gas exchange across the lung itself (like pneumonia, pulmonary embolism or ARDS), while a normal gradient in a hypoxic patient instead points to a problem outside the lungs, like hypoventilation or high altitude. It is a routine part of interpreting arterial blood gas (ABG) results in emergency medicine, pulmonology and critical care, and the equation also underlies altitude physiology calculations for climbers and aviators exposed to lower barometric pressure.

Enter FiO₂, barometric pressure, PaCO₂ and RQ, and this calculator solves the equation in one step to give you alveolar PO₂.