How many atoms are in the human body?
The element-by-element calculation, and how it scales with body size
Matic Broz, PhD8 min
A 70-kilogram (154-pound) adult contains about 7 × 1027 atoms, or 7 octillion: a 7 followed by 27 zeros. We arrive at that figure by converting the mass of each element in a standard reference body into a number of atoms, which gives 6.7 × 1027, and rounding.
The count scales almost directly with body weight, at roughly 1026 atoms per kilogram. A 60-kilogram adult therefore has about 6 × 1027 atoms and a 100-kilogram adult nearly 1028. About 62% of those atoms are hydrogen, 24% oxygen, 12% carbon, and just over 1% nitrogen.
How many atoms are in a human body?
Using the elemental composition of the reference man defined by the International Commission on Radiological Protection (ICRP), we estimate 6.72 × 1027 atoms in a 70-kilogram adult, which rounds to 7 × 1027.[1][2] The ICRP built this reference man, 1.70 meters tall and 70 kilograms, as a standard body for radiation-protection calculations, and its tables give the mass of each element the body contains.
The number is hard to picture. Counting a billion atoms every second, you would need about 210 billion years to finish, roughly 15 times the 13.8-billion-year age of the universe.[3]
Most of these atoms sit in water. About 60% of an average 70-kilogram man's weight is water, or 42 liters.[4] Each water molecule carries three atoms, so those 42 kilograms of water hold about 4.2 × 1027 atoms, close to two thirds of the body's total. Water is also why hydrogen dominates the count, as the next section shows.
Which atoms are most common in the human body?
By number, about 62% of the atoms in the human body are hydrogen, 24% are oxygen, 12% are carbon, and 1% are nitrogen. Together these four elements account for about 99% of the body's atoms, and seven others (sodium, potassium, calcium, magnesium, phosphorus, sulfur, and chlorine) make up most of the remaining 1%.[5]
The ranking looks different by mass. Oxygen and carbon atoms are much heavier than hydrogen atoms, so oxygen supplies 61% of the reference body's mass and carbon 23%, while hydrogen supplies only 10%. Table 1 sets the two views side by side, and its atom shares agree with Figure 1 to within a few tenths of a percentage point.
| Element | Mass (kg) | Share of mass | Atoms | Share of atoms |
|---|---|---|---|---|
| Hydrogen | 7.0 | 10.0% | 4.18 × 1027 | 62.2% |
| Oxygen | 43 | 61.3% | 1.62 × 1027 | 24.1% |
| Carbon | 16 | 22.8% | 8.02 × 1026 | 11.9% |
| Nitrogen | 1.8 | 2.6% | 7.74 × 1025 | 1.15% |
| Phosphorus | 0.78 | 1.1% | 1.52 × 1025 | 0.23% |
| Calcium | 1.0 | 1.4% | 1.50 × 1025 | 0.22% |
| Minor elements | 0.51 | 0.7% | About 1 × 1025 | 0.15% |
| Total | 70.09 | 100% | 6.72 × 1027 | 100% |
Hydrogen leads by atom count because it is the lightest element and because the body contains so much water, whose molecules are two-thirds hydrogen. Calcium shows the opposite pattern. It is the fifth most abundant element by mass, concentrated in bone, but each calcium atom weighs about 40 times as much as a hydrogen atom, so it contributes barely one atom in 450.
Methodology
Our estimate rests on the ICRP 23 elemental composition of the 70-kilogram reference man: 43 kilograms of oxygen, 16 of carbon, 7 of hydrogen, 1.8 of nitrogen, 1 of calcium, 0.78 of phosphorus, and 0.51 of minor elements.[2] For each element, we divide its mass by its molar mass and multiply by Avogadro's constant, the number of entities in one mole:
For hydrogen, 7,000 grams divided by 1.008 grams per mole gives 6,944 moles:
The molar masses are the standard atomic weights from the Commission on Isotopic Abundances and Atomic Weights: 1.008 for hydrogen, 12.011 for carbon, 14.007 for nitrogen, 15.999 for oxygen, 30.974 for phosphorus, and 40.078 for calcium. Avogadro's constant is fixed exactly by the International System of Units.[6][7] Standard atomic weights already average over natural isotopes, so heavy hydrogen and carbon-13 are counted with no further correction.
The six named elements give 6.71 × 1027 atoms. The ICRP groups the remaining 0.51 kilograms as minor elements, mostly potassium, sulfur, sodium, chlorine, and magnesium, plus trace metals such as iron and zinc.[1] Assuming an average atomic weight of 30, between sodium (23) and potassium (39), they add about 1 × 1025 atoms. Even if all of that mass were sodium, the total would change by less than 0.2%, so the result, 6.72 × 1027, is insensitive to this assumption.
Every atom counts once, whether it is bonded in a protein, dissolved as a sodium or chloride ion, or locked in bone mineral. Electrons and subatomic particles are not counted separately.
We start from body mass rather than from the number of cells in the human body. Cells vary enormously in size, and a substantial share of body mass lies outside them, in blood plasma, interstitial fluid, connective tissue, and bone mineral. A cell-based estimate would need an average atom count per cell that nobody has measured, while body composition by element has been measured directly.
How does the number of atoms change with body weight?
For adults of broadly similar body composition, the count scales at about 9.6 × 1025 atoms per kilogram, which is 6.72 × 1027 atoms divided by 70 kilograms. Table 2 applies that rate to common body weights.
| Body weight | Estimated atoms |
|---|---|
| 50 kg (110 lb) | 4.8 × 1027 |
| 60 kg (132 lb) | 5.8 × 1027 |
| 70 kg (154 lb) | 6.7 × 1027 |
| 80 kg (176 lb) | 7.7 × 1027 |
| 100 kg (220 lb) | 9.6 × 1027 |
The same rate covers the newer ICRP reference adults, a 73-kilogram man and a 60-kilogram woman, at about 7.0 × 1027 and 5.8 × 1027 atoms.[2] These are proportional estimates, not measurements, and real bodies depart from the reference composition. In 1,374 measurements of American adults by neutron activation analysis, elements such as nitrogen, calcium, potassium, and sodium varied by up to 20% (one standard deviation) within the same five-year age group, and the average young man was larger than the reference man, with more skeleton, lean tissue, and water.[8]
Composition changes the atoms per kilogram only slightly. Water holds 1.0 × 1026 atoms per kilogram, close to the whole-body rate because water dominates the count. Fat, also rich in hydrogen, packs a little more, while bone mineral, built from heavier calcium and phosphorus, packs fewer. Two adults of the same weight can therefore differ by a few percent, but every adult body holds on the order of 1027 atoms.
How many carbon atoms are in the human body?
A 70-kilogram ICRP reference body contains about 8 × 1026 carbon atoms, or 800 septillion.[2][6][7]
The calculation uses 16 kilograms of carbon:
Carbon sits in fat, protein, carbohydrate, and nucleic acid rather than in water, so two bodies of the same weight can hold quite different amounts of it, with fat the largest variable store. The count still stays near 1027 carbon atoms for an adult, with 8 × 1026 as the result for this stated reference model.
Carbon is less common than hydrogen by atom count, but it supplies the covalent frameworks of proteins, lipids, carbohydrates, and nucleic acids. The body's carbon atoms are spread across an enormous range of compounds rather than stored as elemental carbon. Protein sequences alone use 20 standard amino acids, each with its own carbon skeleton.
How many molecules are in the human body?
There is no accepted total number of molecules in the human body. The body is a changing mixture of water, small compounds, ions, minerals, proteins, nucleic acids, membranes, and other structures, and not all of those substances exist as discrete molecules.
Water is the one large component that can be estimated cleanly. Treating the 42 liters of body water in an average 70-kilogram man as 42 kilograms gives about 1.4 × 1027 water molecules.[4]
The water estimate is not a total molecule count. A water molecule has three atoms, while protein molecules can contain thousands of atoms and a chromosome contains one extremely long DNA molecule. Salts in body fluids are commonly present as ions, and bone mineral forms an extended crystal lattice rather than a collection of separate molecules.
For one defined compound, molecular descriptors can report its formula, atom counts, and molecular weight. A whole human body has no single molecular formula, which is why its atoms, fixed by measured elemental composition, can be counted far more defensibly than its molecules.
