How a human body reaches 7 octillion atoms
The estimate comes from adding up the atoms in the body’s ingredients. A typical adult’s mass is mostly water and carbon-rich molecules such as proteins, fats, and sugars. Chemists can estimate how many atoms are in a given mass of each ingredient using its molecular makeup, then add those estimates together.
The result is dominated by a few elements. Hydrogen contributes the most atoms, in large part because water molecules contain two hydrogen atoms apiece and hydrogen atoms are exceptionally light. Oxygen is also abundant, while carbon is a major building block of the molecules that make up cells. Heavier elements such as calcium and iron matter to the body, but contribute far fewer atoms by number.
A useful estimate, not a personal inventory
Seven octillion is an order-of-magnitude estimate, not a count of the atoms in every person. Body size and composition vary, so a smaller person generally has fewer atoms than a larger one. The precise total also depends on what is counted as part of the body at a particular moment, since people continually take in and release matter through breathing, eating, drinking, and waste.
Nor are those atoms sitting around as separate pieces. They are joined into molecules, which in turn form structures such as cell membranes, muscle fibers, and bone. The striking part is that ordinary body mass, built from familiar elements, contains such an enormous number of extremely small building blocks: a change you can weigh in grams still involves an almost unimaginable number of atoms.
