A–a Oxygen Gradient Calculator (Alveolar–arterial Gradient)
The alveolar–arterial (A–a) oxygen gradient is the difference between the calculated alveolar oxygen tension (from the alveolar gas equation) and the measured arterial PaO₂. It helps localise the cause of hypoxaemia. The normal gradient widens with age, approximately (age ÷ 4) + 4 mmHg on room air.
Quick answer
The A–a oxygen gradient is the calculated alveolar oxygen tension minus the measured arterial PaO₂. Alveolar oxygen is PAO₂ = FiO₂ × (atmospheric pressure − 47) − PaCO₂ ÷ 0.8. The normal gradient rises with age, roughly (age ÷ 4) + 4 mmHg on room air; an elevated gradient points to a problem with gas exchange.
Arterial blood gas inputs
Assumes sea level: atmospheric pressure 760 mmHg, water vapour 47 mmHg, respiratory quotient 0.8.
How to use the A–a gradient calculator
- 1Enter the arterial blood gas values. Enter the FiO₂ (0.21 for room air), the measured arterial PaO₂ and PaCO₂ from the blood gas, and the patient’s age.
- 2Read the alveolar oxygen and gradient. The tool calculates the alveolar PAO₂ from the alveolar gas equation and subtracts the measured PaO₂ to give the A–a gradient.
- 3Compare with the age-expected value. The tool shows the age-expected gradient (about (age/4)+4 mmHg) and whether the patient’s gradient is normal or elevated — interpreted with a clinician.
Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.
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Book a consultationFrequently asked questions
What is a normal A–a gradient?⌄
A normal gradient widens with age. A common room-air estimate of the upper limit of normal is (age ÷ 4) + 4 mmHg — so roughly 5–10 mmHg in young adults and higher in older people. A gradient above the age-expected value suggests impaired gas exchange.
What does a high A–a gradient mean?⌄
A raised A–a gradient indicates that hypoxaemia is due to a problem at the gas-exchange level — such as ventilation-perfusion mismatch, shunt, or a diffusion defect — rather than pure hypoventilation, where the gradient stays normal. A clinician uses it to narrow the cause.
Which values does the equation assume?⌄
It assumes atmospheric pressure 760 mmHg at sea level, water vapour pressure 47 mmHg at body temperature, and a respiratory quotient of 0.8. At altitude the atmospheric pressure is lower, which changes the result.
Is my data stored?⌄
No. The calculator runs entirely in your browser; nothing you enter is sent to a server.
Sources & validation
This calculator reproduces the published A–a gradient score, validated for classifying the cause of hypoxaemia via the alveolar–arterial oxygen gradient.
- Alveolar gas equation and A–a gradient, StatPearls (NCBI Bookshelf): PAO₂ = FiO₂×(Patm−PH₂O) − PaCO₂/R; A–a = PAO₂ − PaO₂; age-expected ≈ (age/4)+4 — verified 2026-06-18
- West JB. Respiratory Physiology: The Essentials — standard derivation of the alveolar gas equation (Patm 760 mmHg, PH₂O 47 mmHg, R 0.8 at sea level) — verified 2026-06-18
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