What does this calculator measure?

It estimates alveolar oxygen partial pressure, or PAO₂, in mmHg.

What are the default atmospheric pressure and RQ?

The defaults are 760 mmHg for sea level and 0.8 for a mixed diet.

Can I calculate PAO₂ at altitude?

Yes. Enter the local atmospheric pressure in mmHg instead of using the sea-level default.

Does this replace a complete A-a gradient interpretation?

No. This tool focuses on PAO₂ and does not provide a full A-a gradient interpretation. It is not medical advice.

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Alveolar Gas Equation (PAO₂ Calculator)

Calculate PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. Defaults: P_atm 760 mmHg (sea level), RQ 0.8. Gives a directional hint vs expected ~100 mmHg. For full A-a gradient interpretation use a-a-gradient-calculator. Derived from StatPearls, LITFL, Cornell PICU, and West. Not medical advice.

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2 examples

Sea level room air: PaCO₂=40, FiO₂=0.21 → PAO₂ ≈ 99.7

PAO₂ = 0.21 × (760 − 47) − 40/0.8 = 0.21 × 713 − 50 = 149.73 − 50 = 99.73 mmHg. This is the textbook ~100 mmHg expected at sea level on room air.

{"result":"Alveolar Gas Equation: PAO₂ = 0.21 × (760 − 47) − 40 / 0.8 = 99.7 mmHg.","metadata":{"input":{"paco2":40,"fio2":0.21,"patm":760,"rq":0.8},"result":{"pao2Alveolar":99.7,"fio2":0.21,"patm":760,"rq":0.8,"pH2O":47,"category":"Normal PAO₂ (sea level, room air)","method":"PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ","model":"StatPearls NBK545153; LITFL; Cornell PICU; West. Not medical advice."}}}

High altitude (Denver): PaCO₂=35, FiO₂=0.21, P_atm=630 → PAO₂ ≈ 78.7

PAO₂ = 0.21 × (630 − 47) − 35/0.8 = 0.21 × 583 − 43.75 = 122.43 − 43.75 = 78.68 mmHg. Lower atmospheric pressure at altitude reduces PAO₂ — a normal PaO₂ drop here is explained by altitude, not lung disease. Hyperventilation (low PaCO₂) partially compensates.

{"result":"Alveolar Gas Equation: PAO₂ = 0.21 × (630 − 47) − 35 / 0.8 = 78.7 mmHg.","metadata":{"input":{"paco2":35,"fio2":0.21,"patm":630,"rq":0.8},"result":{"pao2Alveolar":78.7,"fio2":0.21,"patm":630,"rq":0.8,"pH2O":47,"category":"Reduced PAO₂ due to altitude","method":"PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ","model":"StatPearls NBK545153; LITFL; Cornell PICU; West. Not medical advice."}}}

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5 options

SettingsAdjust formats, ranges, numbers, and modes.5

PaCO₂ (mmHg)numberRequiredArterial partial pressure of CO₂ (mmHg) from ABG.FiO₂ (inspired oxygen fraction)numberRequiredInspired oxygen fraction (0.21 = room air). Range 0.15–1.0.Atmospheric Pressure (mmHg, optional)numberOptionalAtmospheric pressure (mmHg). Default 760 at sea level. Lower at altitude (e.g. Denver ~630).Respiratory Quotient (RQ, optional)numberOptionalRespiratory quotient. Default 0.8 (mixed diet); 1.0 pure carbohydrate; ~0.7 high fat/protein.Decimal PlacesnumberOptional

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Alveolar Gas Equation (PAO₂ Calculator)

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1. Sea-level room-air PAO₂ estimate

2. PAO₂ estimate at Denver altitude

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