How many bacteria are in the human body?
Where the estimate comes from and why the 10:1 ratio was wrong
Matic Broz, PhD7 min
The human body carries about 38 trillion bacterial cells, written as 3.8 × 1013. The figure comes from a 2016 census by Ron Sender, Shai Fuchs and Ron Milo for a 70-kilogram reference man, and nearly all of those bacteria live in the colon. Together they weigh about 0.2 kilograms, roughly 0.3% of body weight.
That makes bacteria about as numerous as the body's own human cells: 1.3 bacteria per human cell in the 2016 census, and close to 1:1 against the newer count of 36 trillion human cells. The often repeated claim that bacteria outnumber us 10 to 1 rests on a 1970s back-of-the-envelope estimate that counted the whole digestive tract as if it were colon.
How many bacteria are in the human body?
A 70-kilogram reference man, defined as 20 to 30 years old and 170 centimeters tall, carries an estimated 38 trillion bacteria, with a standard error of about 25%.[1] In scientific notation that is 3.8 × 1013, or 38 followed by twelve zeros.
The estimate varies more between people than the headline number suggests. Sender and colleagues reported a population variation of 52% among reference-sized adult men. Colon size and bacterial concentration both differ, and a single bowel movement removes roughly a quarter to a third of the colon's contents.[1]
The number is easier to picture against the human population. About 8.2 billion people lived on Earth in 2024,[2] so the bacteria in one body outnumber everyone alive roughly 4,600 times. Multiplying instead, if every person carried the reference-man count, all of humanity would host about 3.1 × 1023 bacteria. Treat that as an order of magnitude: a one-year-old carries about 7 trillion bacteria, a reference woman about 44 trillion.[1]
The total counts bacterial cells, not species. Those cells form a diverse community in which Escherichia coli is only one member.
Are there more bacteria than human cells?
Slightly more, but not by much. Sender and colleagues estimated 38 trillion bacteria and 30 trillion human cells in the reference man, a ratio of 1.3 bacterial cells per human cell with an uncertainty of 25%. They proposed that a ratio of about 1:1 replace the 10:1 and 100:1 values repeated in the literature.[1]
A 2023 census by Hatton and colleagues put the reference man's human cells at about 36 trillion.[3] Dividing the 2016 bacterial estimate by that count, we get 1.06 bacterial cells per human cell, or about 1.1:1.
The 10:1 claim paired rough figures of 100 trillion bacteria and 10 trillion human cells. The bacterial figure was too high for reasons covered in the next section. The human figure rose to 30 trillion once cells were tallied type by type, and almost 90% of them turned out to be red blood cells and platelets, small cells without a nucleus.[1]
That also explains why a 10:1 ratio still turns up. Against only the roughly 3 trillion nucleated human cells, bacteria do outnumber the body's cells about ten to one. Sender and colleagues treat the choice as a question of definition and count red blood cells as genuine cells, which is the convention behind the 1.3:1 figure.[1]
The ratio also shifts with sex, age and body size. Blood volume largely sets the human-cell count, while colon content sets the bacterial count, so groups with less blood relative to colon content carry relatively more bacteria. Across the groups Sender and colleagues modeled, the ratio ranged from 1.3 to 2.3.[1]
| Group | Body weight (kg) | Human cells (trillion) | Bacteria (trillion) | Bacteria per human cell |
|---|---|---|---|---|
| Reference man, 20 to 30 years | 70 | 30 | 38 | 1.3 |
| Reference woman | 63 | 21 | 44 | 2.2 |
| Older adult, 66 years | 70 | 22 | 38 | 1.8 |
| Obese adult | 140 | 40 | 56 | 1.4 |
| Infant, 1 year | 9.6 | 4 | 7 | 1.7 |
| Infant, 4 weeks | 4.4 | 1.9 | 4.4 | 2.3 |
Whichever count is used, bacterial and human cells are numbered in the same tens of trillions in adults.
Where do most bacteria in the human body live?
Almost all of the body's bacteria live in the colon. Every other site holds at least a hundred times fewer, and together they add at most about 1 trillion bacteria to the colon's 38 trillion.[1]
The colon estimate multiplies about 0.4 liters of wet colon contents by a bacterial concentration of 92 billion cells per gram. The concentration is the average of 14 stool studies, used as a proxy for the contents of the colon. Sender and colleagues name that proxy as the largest gap in the estimate, since bacterial density rises along the colon and stool sits at the dense end.[1]
| Body site | Bacteria per mL of content | Content volume (mL) | Upper bound on bacteria |
|---|---|---|---|
| Colon (large intestine) | 1011 | 400 | 1014 |
| Dental plaque | 1011 | under 10 | 1012 |
| Ileum (lower small intestine) | 108 | 400 | 1011 |
| Saliva | 109 | under 100 | 1011 |
| Skin | under 1011 per m² | 1.8 m² of surface | 1011 |
| Stomach | 103 to 104 | 250 to 900 | 107 |
| Duodenum and jejunum (upper small intestine) | 103 to 104 | 400 | 107 |
Saliva and dental plaque hold dense communities, but their small volume keeps the mouth below 1% of the colon's total. The stomach and upper small intestine carry only 1,000 to 10,000 bacteria per milliliter because of stomach acid and the fast flow of their contents.[1]
That gradient is what inflated the old estimate. The 100-trillion figure applied the colon's density of about 100 billion bacteria per gram to a full liter of digestive-tract contents, although only the colon's 0.4 liters are that crowded.[1]
How much do the bacteria in the human body weigh?
About 0.2 kilograms. The bacteria in a 70-kilogram reference man have an estimated wet mass of about 200 grams, roughly 0.3% of body weight, and a dry mass of 50 to 100 grams. The wet figure comes from about 400 grams of colon contents, roughly half of which is bacterial biomass.[1]
Older claims of 1 to 3 kilograms of bacteria rested on the same inflated count that produced the 10:1 ratio. The 200-gram estimate also checks out from the other direction: an average gut bacterium has a wet mass of about 5 picograms, and 38 trillion of them weigh about 0.2 kilograms.[1]
Bacteria can match human cells in number and still weigh so little because they are much smaller. That 5-picogram gut bacterium is several times the 1-cubic-micrometer textbook bacterium, yet a red blood cell, among the smallest human cells, has a volume of about 100 cubic micrometers, and fat and muscle cells usually exceed 10,000.[1] Taking cells to be about as dense as water, a red blood cell holds the volume of about 20 gut bacteria, and a typical fat or muscle cell more than 2,000.
Human cells show the same split between count and mass. Red blood cells make up 84% of human cells by number in the 2016 census, while fat and muscle cells, about 0.2% of the count, account for 75% of cell mass.[1] Across all cell types, human cell sizes span about seven orders of magnitude.[3]
Bacteria sit at the light end of that scale: as numerous as the body's own cells, packed into less than half a liter of colon contents, and together about as heavy as a large apple.
