By mass, human cells consist of 65-90% water (H2O), and a significant portion is composed of carbon-containing organic molecules. Oxygen therefore contributes a majority of a human body's mass, followed by carbon.

99% of the mass of the human body is made up of the six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Oxygen 65 %
Carbon 18 %
Hydrogen 10 %
Nitrogen 3 %
Calcium 1.5 %
Phosphorus 1.2 %
Potassium 0.2 %
Sulfur 0.2 %
Chlorine 0.2 %
Sodium 0.1 %
Magnesium 0.05 %
Iron, Cobalt, Copper, Zinc, Iodine <0.05 % each
Selenium, Fluorine <0.01 % each