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The percentage of oxygen in the atmosphere is 21 percent, according to NASA.gov. I was trying to make a mathematical calculation about oxygen consumption of humans, when I realised that there is a gap in my understanding of the above mentioned fact. Is 21% of the MASS of the atmosphere that of oxygen, or is 21% of the VOLUME of the atmosphere that of oxygen? The mass is not evenly distributed in the atmosphere, and the volume of it is fixed. And for that matter, I have another question, how exactly is the percentage of oxygen in the atmosphere calculated?

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  • $\begingroup$ Oxygen can be "measured" indirectly using spectrometry and a set of known references with calibrated gas. See the Scripps Program here: scrippso2.ucsd.edu/what-we-do.html $\endgroup$
    – f.thorpe
    Jan 7 at 7:17

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The atmosphere is 20.95% Oxygen by volume, on a dry basis (i.e. without any water vapor). See here for detailed composition information.

Your second question, how is the oxygen percentage calculated, is misguided I believe. The composition of the atmosphere is measured with gas analyzers, not calculated.

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    $\begingroup$ Thank you for considering my questions and answering them, I appreciate it! $\endgroup$ Jan 7 at 2:26

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