The human body consists of close to 100 trillion cells,[1] the basic unit of life.
By mass, human cells consist of 65–90% water (H2O), says Wikipedia.
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.