What percentage of the earth's atmosphere is oxygen?

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. read more

Life itself, in other words, turned earth’s vacuum back up again. By 2 billion years ago, oxygen levels were down to about 0.01 percent of current levels. Life and earth have continued to twiddle the oxygen knob over the past two billion years. read more

Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. read more