Approximately what fraction of the Earth's air is made up of oxygen?

Approximately one-fifth of Earth’s air is made up of oxygen. By volume, dry air contains about 20.95% oxygen, which is close to 21%, or one part oxygen for every four parts of other gases.

Most of the remaining atmosphere is nitrogen, at about 78%. Argon makes up roughly 0.93%, while carbon dioxide and several trace gases account for most of the small remainder. These proportions describe dry air; water vapour varies considerably with location, temperature and weather.

The oxygen in the atmosphere is continually involved in chemical and biological cycles. Green plants, algae and cyanobacteria release oxygen during photosynthesis, while animals, fungi and many microbes consume it during respiration. Fires and many chemical reactions also use atmospheric oxygen.

A common mistake is to think that oxygen is the largest component of air. Nitrogen is actually much more abundant. Another mix-up is confusing atmospheric oxygen with the oxygen bound inside water or rocks. The approximately 21% figure refers to molecular oxygen gas, O₂, floating freely in the atmosphere and available for breathing and combustion.