Which element is the most common in the human body?

Oxygen is the most common element in the human body by mass.

It makes up roughly 65% of an average human body’s mass, largely because the body contains substantial water. Oxygen is also part of carbohydrates, proteins, fats, DNA, and other molecules. Carbon is usually second at about 18.5%, followed by hydrogen and nitrogen.

A common mix-up comes from counting atoms instead of measuring mass. Hydrogen is the most abundant element by number of atoms, because water contains two hydrogen atoms for every oxygen atom and hydrogen atoms are extremely light. Oxygen remains the clear leader when the question means overall body weight, which is the standard interpretation in general science quizzes.

Oxygen also has a vital biological role: cells use it during aerobic respiration to help release energy from nutrients. The oxygen we breathe is not simply stored as free O₂ throughout the body; most of the body’s oxygen is chemically bound in water and organic compounds.