A normal gradient with low oxygen suggests hypoventilation or altitude; a raised gradient points to lung problems such as V/Q mismatch.
How to use the A-a Gradient Calculator
- FiO₂
- PaCO₂
- PaO₂
- Atmospheric pressure
- Age
Results update as you type, and every field accepts the units shown in its menu.
How it works
Alveolar oxygen is calculated from inspired oxygen, pressure and carbon dioxide, then arterial oxygen is subtracted.
Inputs explained
| Field | What to enter |
|---|---|
| FiO₂ | In % |
| PaCO₂ | In mmHg |
| PaO₂ | In mmHg |
| Atmospheric pressure | In mmHg |
| Age | In years |
What each result means
- Alveolar oxygen (PAO₂) = [FiO₂]÷100 × ([Atmospheric pressure] – 47) – [PaCO₂]÷0.8
- A-a gradient = [Alveolar oxygen (PAO₂)] – [PaO₂]
- Expected for age = [Age]÷4 + 4
Example values
How the main result changes when FiO₂ changes and everything else stays at the example values above:
| FiO₂ (%) | A-a gradient (mmHg) |
|---|---|
| 21 | 9.7 |
| 26.25 | 47.2 |
| 31.5 | 84.6 |
| 42 | 159.5 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
PAO₂ = FiO₂ × (Patm − 47) − PaCO₂ ÷ 0.8A-a gradient = PAO₂ − PaO₂Expected ≈ age ÷ 4 + 4Worked example
Inputs — FiO₂: 21 %; PaCO₂: 40 mmHg; PaO₂: 90 mmHg; Atmospheric pressure: 760 mmHg; Age: 40 years.
Results — Alveolar oxygen (PAO₂) = 99.7 mmHg; A-a gradient = 9.7 mmHg; Expected for age = 14 mmHg.
Frequently asked questions
Why 47 mmHg?
It is the water vapour pressure at body temperature.
Does the formula change with oxygen therapy?
The gradient rises with higher FiO₂, so interpret with care.
What formula does the A-a Gradient Calculator use?
PAO₂ = FiO₂ × (Patm − 47) − PaCO₂ ÷ 0.8.