Alveolar Gas Equation Calculator

Determines the partial pressure of alveolar oxygen that reflects the ventilation process.

In the text below the form there is more information about the variables and the equation used.

The alveolar gas equation (AGE) reflect the relationship between the partial pressure of oxygen in the inspired air and that from the alveoli.

Alveolar oxygen is used in calculating the alveolar-arterial (A-a) gradient of oxygen and the amount of right-to-left cardiac shunt.

The alveolar oxygen equation is:

pAO2 = FIO2 x (PATM – PH2O) – (paCO2 / RQ)

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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Variables and formula

The alveolar gas equation dates from back in 1946 and refers to the partial alveolar pressure of oxygen during the alveolar ventilation exchange.

The partial pressure of oxygen (pAO2) in the pulmonary alveoli is further used in determinations such as the alveolar-arterial (A-a) gradient of oxygen and in the right-to-left cardiac shunt.

The five variables taken into account are described in the table below:

AGE variable Name Definition Normal value
FIO2 Fraction of inspired gas that is oxygen Amount of oxygen content in the inspired air 21%, 0.21 in normal air
PATM Atmospheric pressure The pressure exerted by the weight of the atmosphere (at sea level) 760 mmHg
PH2O Saturated vapor pressure of water Water pressure at body temperature and at normal atmospheric pressure 47 mmHg at 37 degrees Celsius
paCO2 Arterial partial pressure of carbon dioxide Pressure of CO2 during the alveolar exchange 40 mmHg
RQ Respiratory exchange ratio Respiratory quotient, meaning the ratio between pO2 and pCO2 0.8

The alveolar gas equation is:

pAO2 = FIO2 x (PATM – PH2O) – ((paCO2 x (1 - FIO2 x (1 - RQ))) / RQ)

If this is true:

FIO2 x (1 - RQ) ≪1

Then AGE becomes:

pAO2 = FIO2 x (PATM – PH2O) – (paCO2 / RQ)

In order for the above equation to be applicable, there are several assumptions to be made:

■ The inspired gas needs to be “pure” and not contain carbon dioxide or water, whilst the other gases (e.g. nitrogen) are in equilibrium with their dissolved states in the blood;

■ Alveolar carbon dioxide is in equilibrium with the arterial blood, meaning that the alveolar and arterial pCO2s are equal;

■ The alveolar gas is saturated with water;

■ The equation respects the ideal gas law.


About alveolar oxygen pressure

pAO2 is determined by two opposing processes that ensure the movement of oxygen towards the alveolus and the extraction of carbon dioxide towards the venous blood.

The entry of oxygen in the alveolus (the first process) is influenced by:

■ The rate of alveolar ventilation;

■ The starting partial pressure of oxygen in the external environment.

The second process refers to the pulmonary diffusion rate of the oxygen in the capillaries.

When arterial oxygenation is dysfunctional, serious pulmonary conditions develop.



1. Cruickshank S, Hirschauer N. The alveolar gas equation. Oxford Journals Medicine & Health BJA: CEACCP. 2004; 4(1P), 24-27.

2. Carroll GC. Misapplication of alveolar gas equation. N Engl J Med. 1985; 312(9):586.

3. Curran-Everett D. A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation. Adv Physiol Educ. 2006; 30(2):58-62.

4. Conkin J. Equivalent Air Altitude and the Alveolar Gas Equation. Aerosp Med Hum Perform. 2016; 87(1):61-4.

Specialty: Pulmonology

System: Respiratory

Objective: Determination

Type: Calculator

No. Of Variables: 5

Article By: Denise Nedea

Published On: June 9, 2017

Last Checked: June 9, 2017

Next Review: June 9, 2023