A-a Gradient Calculator
Calculate the oxygen gradient. Check the assumptions. Interpret in context.
Your Oxygen Gradient
Not above age reference
Room air comparison — not a diagnosis.
Age/4 + 4 · approximate adult upper limit on room air at sea level.
Alveolar vs. Arterial Oxygen
Pressure (mmHg)
Difference: 9.7 mmHg
FiO₂ 21% · PB 760 mmHg · R 0.80
An A-a gradient alone cannot rule out serious illness.
Clinical Interpretation Context
If hypoxemia is present, these patterns may help frame clinical assessment.
May be compatible with hypoventilation or reduced inspired oxygen.
Requires full ABG and clinical context.
May suggest V/Q mismatch, shunt or diffusion limitation.
Does not identify a specific cause.
The age rule is approximate and is not a validated threshold for high FiO₂. A non-elevated result does not exclude disease.
Severe breathlessness, blue lips or confusion: seek urgent medical care. Do not delay care to use this tool.
Calculation & Assumptions
PAO₂ = FiO₂ × (PB − 47) − PaCO₂ / R
A-a gradient = PAO₂ − PaO₂
FiO₂ is converted from percent to a fraction.
Common simplified equation · steady-state assumptions.
Limitations & unusual results
The simplified alveolar gas equation assumes humidified gas at 37°C, negligible inspired CO₂ and arterial CO₂ approximating alveolar CO₂. R is assumed unless independently known. High FiO₂ and non-steady-state conditions limit its accuracy. Use contemporaneous arterial measurements and a known FiO₂. Do not substitute venous PO₂ or SpO₂ for arterial PaO₂. The adult age rule is compared only at FiO₂ 20.9–21.1%, PB 760 mmHg and R 0.80; it is not an individualized diagnostic limit. Negative gradients are retained with a review warning; a negative computed alveolar pressure is rejected. Display rounding does not change the calculation. Pressure units apply to blood gases and results; PB stays explicitly in mmHg. Enter local station pressure, not altitude-adjusted weather pressure. No clinical diagnosis, oxygen dose, treatment plan or patient-specific prognosis is generated.
Frequently Asked Questions
Can I enter SpO₂ instead of PaO₂?
No. SpO₂ is an oxygen saturation percentage from a pulse oximeter; PaO₂ is an arterial oxygen partial pressure measured in a blood gas sample. They are different quantities and are not interchangeable. Venous PO₂ is also unsuitable for this calculation.
Does the age limit apply on oxygen?
The displayed age/4 + 4 rule is an approximate adult room-air reference. This calculator withholds that comparison on supplemental oxygen, at custom barometric pressure, with custom R, or below age 18. A clinician must interpret the actual gradient alongside the full ABG and patient presentation.
What if the gradient is negative?
The signed result is retained rather than clamped to zero. Review the arterial sample, units, FiO₂ at sampling, barometric pressure, R and steady-state assumptions. The calculator cannot establish whether a discrepancy is due to measurement, assumptions or physiology. Do not change treatment based on this value alone.
Sources & References
Gas exchangeAlveolar gas equation & context.
Arterial blood gasesClinical education resources.
Respiratory physiologyPublished oxygen-gradient references.Source attribution does not imply institutional endorsement.
Calculate the oxygen gradient. Check the assumptions. Interpret in context.