Alveolar–Arterial (A–a) Gradient
A–a gradient for assessing gas exchange.
What this calculator does
This tool calculates the alveolar–arterial (A–a) gradient — the difference between the oxygen pressure in the lung air sacs and in the arterial blood. It helps assess how well the lungs are oxygenating the blood.
What your result means
A higher gradient can suggest a problem with gas exchange. The result is interpreted along with other blood gas and clinical data. Only a healthcare provider can interpret what it means for you.
This is not medical advice. A healthcare provider should interpret results in your context.
PaO₂ (mmHg)
PaCO₂ (mmHg)
FiO₂ (decimal, e.g. 0.21)
Key terms
- A–a gradient
- Alveolar–arterial oxygen gradient; difference between oxygen pressure in the lung alveoli and in arterial blood.
- PaO₂
- Partial pressure of oxygen in arterial blood (mmHg).
- PaCO₂
- Partial pressure of carbon dioxide in arterial blood (mmHg).
- FiO₂
- Fraction of inspired oxygen (e.g. 0.21 for room air).
Related calculators
- PaO₂/FiO₂ RatioRespiratory & blood gas
- Oxygen Content (CaO₂)Respiratory & blood gas
- BMI CalculatorBody & metabolic
- BSA Calculator (Mosteller)Body & metabolic
- Ideal Body Weight (Devine)Body & metabolic
- Adjusted Body WeightBody & metabolic
- GFR (CKD-EPI)Renal & lab
- GFR (MDRD 4-variable)Renal & lab
This tool is for educational and reference use only. It does not constitute medical advice. Always consult a healthcare professional for medical decisions.