A–a Gradient Calculator
How much oxygen is lost between the alveolus and the artery — the classic test for where a hypoxemia problem lives.
Written by Apex Respiratory Editorial Team
Accepts 0.21–1.0 or 21–100.
Sea level ≈ 760; adjust for altitude.
Sets the expected normal.
Enter PaO₂, PaCO₂, and FiO₂ to calculate the gradient.
Reading the gradient
An elevated gradient means the lungs themselves are failing to transfer oxygen: V/Q mismatch, shunt, or diffusion impairment. A normal gradient with hypoxemia means the lungs are fine but the inputs aren’t — hypoventilation or low inspired oxygen.
The gradient widens naturally with age (expected ≈ age/4 + 4) and with FiO₂, which is why a “normal” value on 100% oxygen can still exceed 100 mmHg. Trend it at a consistent FiO₂ where possible.
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Gas exchange chapters.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2020.