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How much of Earth’s atmosphere is oxygen by volume?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — oxygen group element Modern version of the periodic table of the elements (printable).

Oxygen accounts for approximately 20.946 percent of Earth’s atmosphere by volume. While some sources round this figure to 21 percent, the more precise measurement is 20.946 percent.12

This concentration is maintained primarily through photosynthesis, a biological process in which green plants and other organisms assimilate carbon dioxide and release free oxygen. Over geological time, this process has been the most significant factor in controlling the composition of the atmosphere.23