Aa Gradient Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

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A lung or oxygenation value can be easy to misread when the formula is hidden. Aa Gradient Calculator makes the calculation explicit, so you can see what the result represents—and what it does not.

What this calculator does

Aa Gradient Calculator uses Age, FiO₂, PaCO₂, PaO₂, and Atmospheric pressure to produce the alveolar–arterial o₂ gradient used by this calculator. It only uses the fields shown in this calculator; it does not silently add tests, diagnoses, symptoms, or clinical variables that are absent from the interface. A–a gradient inputs, using respiratory quotient 0.8 internally; reports expected age-related A–a gradient.

How to use it

Enter or select Age, FiO₂, PaCO₂, PaO₂, and Atmospheric pressure. Use measurements from the same respiratory assessment and confirm whether oxygen values are percentages, fractions, or pressures. If an input is unknown, it is better to leave the calculation unresolved than to invent a value.

How the calculation works

The alveolar oxygen pressure is estimated with the alveolar gas equation: FiO2 × (atmospheric pressure − 47) − PaCO2/0.8. The A–a gradient is then estimated alveolar PO2 minus measured arterial PaO2. Age is used for an expected-gradient reference rather than for the primary subtraction.

Example

Using the calculator’s prefilled demonstration values, the primary output is 9.73 mmHg, labeled “Alveolar–arterial O₂ gradient”; Estimated PAO₂ is 99.7 mmHg and Expected gradient for age is 14 mmHg. That sample is useful for checking the calculation path, but real interpretation should use verified measurements from the same clinical or study context.

How to interpret the result

Read alveolar–arterial o₂ gradient in the context of the pulmonary measurement or scoring system used. Trends, measurement quality, oxygen/ventilator settings, and the disease being evaluated usually matter more than an isolated decimal value.

Limitations and notes

Respiratory measurements depend on technique, calibration, oxygen/ventilator settings, altitude, timing, and the patient population in which a rule was validated. A single calculated value should be interpreted with symptoms, examination, serial measurements, imaging, and the clinical question that prompted the test.