Which Is the Most Common Element in the Human Body?

Oxygen is the most abundant element in the human body by mass, comprising roughly 65 %. Most of it is found in water (H₂O), which makes up about 60 % of body weight. Carbon is second at about 18 %, forming the backbone of organic molecules. Hydrogen (10 %) and nitrogen (3 %) round out the top four. By atom count, however, hydrogen is the most common because water contains two hydrogen atoms for every oxygen atom. Trace elements like iron, zinc, and iodine are essential despite making up less than 0.1 % of body mass.