How many cells are in the human body?
Three whole-body censuses, what they count, and why they disagree
Matic Broz, PhD10 min
An adult human body contains roughly 30 to 37 trillion human cells. The three whole-body censuses most often quoted arrive at 37.2 trillion (2013), 30 trillion (2016) and 36 trillion (2023), each for a reference man of about 70 kilograms. The newest and most detailed also puts a reference woman at about 28 trillion cells and a 10-year-old child at about 17 trillion.
None of these numbers comes from counting cells one by one. Each adds up estimates for individual tissues, and most of the total turns out to be red blood cells, which are tiny. The muscle and fat cells that make up most of the body's weight are a sliver of the count, so the number of cells in a body says surprisingly little about how heavy it is.
How many cells are in the human body?
Current estimates put an adult at 30 to 37 trillion human cells. Bianconi and colleagues summed published counts organ by organ and arrived at 37.2 trillion for a 70-kilogram, 30-year-old man.[1] Sender, Fuchs and Milo revisited the largest categories three years later and lowered the total to 30 trillion.[2] The most detailed census, published by Hatton and colleagues in 2023, drew on more than 1,500 sources for about 400 cell types and estimated 36 trillion.[3]
| Study | How the cells were counted | Human cells |
|---|---|---|
| Bianconi et al. (2013) | Published counts summed across organs and cell types | 37.2 trillion |
| Sender et al. (2016) | Recalculated counts for the most abundant cell types | 30 trillion |
| Hatton et al. (2023) | 1,264 cell groups across 60 tissue systems | 36 trillion |
The 2023 figure is the most complete, but the range is the honest answer. Hatton and colleagues found that published counts for a single cell type usually differ by a factor of two to four between studies, and occasionally by twentyfold.[3] Sender and colleagues also estimated that the total varies by about 14% among healthy adults of the same reference size.[2]
Why do estimates of the number of cells differ?
The 2013 and 2016 estimates agree closely on red blood cells, about 26 and 25 trillion, and part ways over three groups of nucleated cells. Sender and colleagues replaced the old assumption of ten glial cells per neuron with a ratio near one to one, and recalculated two more groups from tissue measurements: the endothelial cells that line blood vessels and the fibroblasts of the skin. Those three groups fell from 7.5 trillion to 0.9 trillion, most of the 7-trillion gap.[2]
The 2023 census raised the total again because it covers more of the body in finer detail. Hatton and colleagues divided the body into 60 tissue systems rather than about 20, and their counts of nucleated blood cells are several times higher than earlier ones. They put the body's lymphocytes near 2 trillion, against the 500 billion often cited.[3] That part is still unsettled: a census of immune cells published the same year found 1.8 trillion immune cells in total, of which lymphocytes made up 40%.[4]
The simplest shortcut, dividing body size by the size of an average cell, does not settle the question either. Bianconi and colleagues tried it both ways. Dividing 70 kilograms by an average cell mass of 1 nanogram gives 70 trillion cells, while dividing body volume by an average cell volume of 4 picoliters gives about 15 to 16 trillion.[1]
The two shortcuts disagree because a body is not built from average cells. About 25 of the reference man's 70 kilograms are not cells at all but extracellular water, protein and bone mineral, and cell sizes span more than seven orders of magnitude, from red blood cells to the longest muscle fibers.[3] Only a tissue-by-tissue count can handle that spread.
Which cells make up the count?
A 70-kilogram man has about 25 trillion red blood cells, more than any other cell type.[2] In the 2023 census, red blood cells and platelets, neither of which has a nucleus, account for about 29 of the 36 trillion cells. That leaves roughly 7 trillion nucleated cells, about half of them white blood cells.[3] Only nucleated cells carry a nuclear genome, so this smaller number is the one that sets the total length of DNA in the body.
| Cells | Estimated number |
|---|---|
| Red blood cells | 25 trillion |
| Red blood cells and platelets together | About 29 trillion |
| All nucleated cells | About 7 trillion |
| White blood cells and other immune cells | 1.8 to 3.4 trillion |
| Endothelial cells lining blood vessels | 0.6 trillion |
| Neurons in the brain | 88 billion |
| Glial cells in the brain | 53 billion |
| Fibroblasts in the skin's dermis | 26 billion |
Mass tells the opposite story. The reference man carries about 45 kilograms of cells, and about half of that is skeletal muscle fibers, even though they make up less than 0.002% of nucleated cells. Fat cells add roughly 12 kilograms more.[3]
Hatton and colleagues found that this trade between size and number holds across the whole body. Each doubling of cell size comes with roughly a halving of cell count, so every size class, from red blood cells to muscle fibers, holds a similar share of the body's cellular mass.[3]
How many cell types are in the human body?
There is no fixed count of human cell types, but modern cell atlases distinguish several hundred. The Tabula Sapiens atlas classified 475 distinct cell types from nearly 500,000 cells sampled across 24 tissues and organs.[5]
The dedicated guide to human cell types explains why published counts range from hundreds to thousands, how a cell type differs from a temporary state, and how single-cell atlases classify cells.
How does the cell count change with sex, age and body size?
A smaller body has fewer cells, but not in proportion to its weight. Hatton and colleagues scaled their male census to a 60-kilogram woman and a 32-kilogram 10-year-old child and estimated about 28 trillion and 17 trillion cells.[3] Sender and colleagues estimated 21 trillion for a reference woman, 4 trillion for a 1-year-old infant, 22 trillion for a 66-year-old man and 40 trillion for a man weighing 140 kilograms.[2]
| Reference body | Body mass | Human cells | Census |
|---|---|---|---|
| Adult man | 70 kg | 36 trillion | Hatton et al. (2023) |
| Adult man | 70 kg | 30 trillion | Sender et al. (2016) |
| Adult woman | 60 kg | 28 trillion | Hatton et al. (2023) |
| Adult woman | 63 kg | 21 trillion | Sender et al. (2016) |
| Man aged 66 | 70 kg | 22 trillion | Sender et al. (2016) |
| Adult man with obesity | 140 kg | 40 trillion | Sender et al. (2016) |
| 10-year-old child | 32 kg | 17 trillion | Hatton et al. (2023) |
| 1-year-old infant | 9.6 kg | 4 trillion | Sender et al. (2016) |
| 4-week-old infant | 4.4 kg | 1.9 trillion | Sender et al. (2016) |
Because red blood cells dominate, these totals largely follow the amount of blood in each body. Doubling body weight from 70 to 140 kilograms adds only about a third more cells, partly because fat and muscle grow by enlarging existing cells: Hatton and colleagues note that counts of fat cells and muscle fibers stay fairly fixed through obesity and muscle loss.[3]
The two censuses disagree more for women than for men, 21 trillion against 28 trillion. Hatton and colleagues built their female and child estimates by scaling the far better documented male model and say their confidence is greatest for the man.[3]
How many bacteria are in the human body compared with human cells?
A 70-kilogram reference man also carries about 38 trillion bacteria, almost all of them in the colon. Against Sender's own 30 trillion human cells that is 1.3 bacteria per human cell, and against the 2023 census of 36 trillion it is close to one to one.[2] Bacterial cells are small, so together they weigh only about 0.2 kilograms.[2]
The long-repeated claim of ten bacteria for every human cell traces back to a rough estimate from 1972 that assumed about 100 trillion gut bacteria. Sender and colleagues showed that it used too large a volume of gut contents.[2]
How many cells die in the human body per day?
About 330 billion human cells die and are replaced each day in a 70-kilogram reference man, equivalent to roughly 3.8 million cells per second.[6]
The estimate is 0.33 ± 0.02 trillion cells and 80 ± 20 grams of cellular mass per day. Close to 90% of the turnover by cell number comes from blood cells, including about 210 billion red blood cells a day; gut epithelial cells account for much of the remainder.[6]
Against the 36-trillion census, we calculate that 330 billion is about 0.9% of all human cells. Cell death and production balance in a healthy steady state, but turnover rates differ sharply between tissues: blood and gut cells renew quickly, while many neurons and heart muscle cells last for years or decades.
How many viruses are in the human body?
The human virome is thought to contain roughly 10 trillion virus particles, but no precise whole-body count exists and the total varies greatly among people.[7]
“Viruses in the body” can mean several different things: free virus particles, viruses replicating inside human cells, bacteriophages that infect the body's bacteria, or latent viral DNA. Most particles in the healthy human virome are thought to be bacteriophages rather than viruses attacking human cells.[7]
Particle count and viral diversity are also different questions. A 2023 study of eukaryotic DNA viruses in nine organs detected 17 viruses across 31 people, with a mean of 6.7 detected virus types per person. It examined a defined panel and does not represent every virus or every body site.[8]
Some viral sequences have become part of our chromosomes over evolutionary time. They contribute to estimates of viral DNA in the human genome, but they are not 10 trillion active infections. This mix of particles, latent infections, bacteriophages and inherited sequences is why the viral total should be read as an order-of-magnitude estimate, much rougher than the count of the body's own cells.
