Respiratory

A-a Gradient Calculator

Alveolar minus arterial oxygen; separates hypoxemia from hypoventilation (normal gradient) from a lung problem (wide gradient).

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

Turn on JavaScript to use this calculator, or open it in the web app.

Formula

A-a Gradient = PAO₂ - PaO₂
PAO₂ = (Atmospheric Pressure - Water Vapor Pressure) × FiO₂ - (PaCO₂ / 0.8)

Reference: Normal about age ÷ 4 + 4 on room air

How it is used

• Normal A–a gradient is roughly age ÷ 4 + 4 on room air; it widens on supplemental oxygen.
• Low PaO₂ with a normal gradient: hypoventilation or low inspired oxygen (altitude).
• Low PaO₂ with a high gradient: a lung problem (V/Q mismatch, shunt or diffusion defect).

Same formula as the app. Open it in the web app.

Related one-pagers

• ABG Interpretation in 6 StepsA–a gradient = PAO₂ − PaO₂; normal about age ÷ 4 + 4
• Ventilator Modes and Settings

Related calculators

• Alveolar Gas Equation (PAO₂)
• Arterial Oxygen Content (CaO₂)
• Minute Ventilation
• P/F Ratio (PaO₂/FiO₂)
• Predicted Body Weight (PBW)
• SpO₂/FiO₂ Ratio
• Ventilator Rate for a Target PaCO₂

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your local protocols and medical direction. Not a substitute for clinical judgment.