A-a gradient calculator (alveolar-arterial)
Enter arterial blood gas results to see whether low oxygen comes from reduced breathing or from a gas exchange problem.
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How to use it
- Enter inspired oxygen fio2, pao2, paco2, age and barometric pressure.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
PAO2 = FiO2 × (barometric pressure - 47) - PaCO2 ÷ 0.8. Gradient = PAO2 - PaO2. Expected value ≈ age ÷ 4 + 4.
Example
Inputs: Inspired oxygen FiO2 = 21 %, PaO2 = 90 mmHg, PaCO2 = 40 mmHg, Age = 40 years, Barometric pressure = 760 mmHg. Result: A-a gradient: 9.7 mmHg; Alveolar oxygen PAO2: 99.7 mmHg; Expected for age (room air): 14 mmHg.
Limits and caveats
- It assumes a respiratory quotient of 0.8 and water vapor pressure of 47 mmHg at 37 °C.
- The age based normal value applies to room air; the gradient rises normally with higher FiO2.
- An educational estimate that does not replace assessment by a doctor or pharmacist.
Questions about this tool
What is the A-a gradient used for?
It separates low oxygen from reduced breathing, where the gradient is normal, from lung problems such as embolism or pneumonia, where it is raised.
What barometric pressure should I enter?
760 mmHg at sea level. It is lower at altitude, so enter your local pressure if you know it.
What is a normal value?
Roughly age ÷ 4 + 4 on room air, so about 14 mmHg at age 40.