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Alveolar–Arterial Oxygen Gradient Calculator — result sheet
Estimate alveolar oxygen pressure with the simplified alveolar gas equation and subtract arterial oxygen pressure to calculate the A–a gradient.
Inputs used
Results
Visual chart
Breakdown
Calculation steps
Returned data table
Formula and methodology
Formula: PAO₂ = (barometric pressure−water-vapor pressure)×FiO₂−PaCO₂/RQ; A–a gradient = PAO₂−PaO₂.
The calculator keeps the atmospheric, water-vapor, inspired-oxygen, carbon-dioxide, respiratory-quotient, and arterial-oxygen inputs visible. It first estimates alveolar oxygen pressure, then subtracts the entered arterial oxygen pressure. This is a physiology worksheet, not a respiratory diagnosis or treatment tool.
This result follows the calculator's declared inputs, precision, validation boundaries, and model limits.
Input contract
- Inspired oxygen concentration — %; minimum 0.01; maximum 100
- Barometric pressure — mmHg; minimum 0.01; maximum 2000
- Water-vapor pressure — mmHg; minimum 0; maximum 200
- Arterial carbon-dioxide pressure — mmHg; minimum 0; maximum 500
- Respiratory quotient — minimum 0.01; maximum 5
- Arterial oxygen pressure — mmHg; minimum 0; maximum 1000
Worked example
| Input | Value |
|---|---|
| Inspired oxygen concentration | 21 |
| Barometric pressure | 760 |
| Water-vapor pressure | 47 |
| Arterial carbon-dioxide pressure | 40 |
| Respiratory quotient | 0.8 |
| Arterial oxygen pressure | 95 |
Estimated PAO₂ is 99.73 mmHg and the A–a gradient is 4.73 mmHg.
Assumptions and limits
- Pressures use mmHg and inspired oxygen is entered as a percentage converted to a fraction.
- The simplified alveolar gas equation is used with the entered respiratory quotient.
- The measured arterial oxygen and carbon-dioxide values come from a comparable blood-gas context.
- Barometric pressure exceeds water-vapor pressure and the respiratory quotient is positive.
- Steady-state conditions, gas exchange assumptions, sampling quality, and supplemental-oxygen delivery are not validated.
- Clinical interpretation depends on the full patient context and professional assessment.
Calculator note
Source and methodology
Use the official WorldCalculate methodology policy for the source, formula, precision, and boundary standards behind this calculator.
Planning estimate, not financial, medical, legal, or professional advice. © WorldCalculate — reuse with attribution. Built and curated by Hassan ALRowaie.
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