Which element accounts for approximately 65% mass of the human body?

Oxygen makes up approximately 65% of the mass of the human body. The approximate amounts of the other elements in the human body are: Carbon 18.5 %, Hydrogen 9.5% and Calcium 1.5%.

Almost 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Only about 0.85% is composed of another five elements: potassium, sulfur, sodium, chlorine, and magnesium. All 11 are necessary for life. The remaining elements are trace elements, of which more than a dozen are thought on the basis of good evidence to be necessary for life. All of the mass of the trace elements put together (less than 10 grams for a human body) do not add up to the body mass of magnesium, the least common of the 11 non-trace elements.