Most of the human body is made up of water, H2O, with cells consisting of 65-90% water by weight. Therefore, it isn’t surprising that most of a human body’s mass is Oxygen.

Carbon, the basic unit for organic molecules, comes in second. 99% of the mass of the human body is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

Image above shows all the elements present in human body.

The four most abundant elements in the human body – hydrogen, oxygen, carbon and nitrogen account for more than 99 per cent of the atoms inside you. They are found throughout your body, mostly as water but also as components of biomolecules such as proteins, fats, DNA and carbohydrates.

Atomic number	Element	Fraction of mass	Mass (kg)	Atomic percent	Group
8	Oxygen	0.65	43	24	16
6	Carbon	0.18	16	12	14
1	Hydrogen	0.10	7	62	1