How many bacteria live in the human body?
Guess before you scroll — the answer is below
The answer
38,000,000,000,000 bacteria
Roughly 38T.
The best current estimate is about 38 trillion bacteria, most of them in the colon — roughly the same order as the number of human cells, not the ten-to-one ratio that circulated for decades. That older figure traced back to a single 1972 back-of-envelope estimate that nobody checked.
How to get close to this number
Anchor on the number of human cells and then decide on a ratio. If you have any sense of how many cells a human body contains, the bacterial count is within striking distance of it. The explanation states the conclusion plainly: the two are roughly the same order of magnitude. Anchoring on the better-known quantity and adjusting is usually safer than estimating the unfamiliar one directly.
The trap here is a specific piece of folklore. For decades the claim that bacteria outnumber human cells ten to one appeared everywhere, and as the explanation says, it traced to a single back-of-envelope calculation from 1972 that nobody rechecked for forty years. If your guess was ten times the human cell count, that is not bad reasoning — it is a well-travelled wrong number, and the correction is the most interesting thing on this page.
It is also worth knowing where they are. The overwhelming majority live in the colon, so the "bacteria in the body" figure is really a statement about one organ. And the modern estimate comes with wide uncertainty and varies substantially between individuals, between meals and after antibiotics — it describes a typical adult, not a fixed property of human beings.
Where this number comes from
| Source | Sender, Fuchs & Milo, PLOS Biology |
|---|---|
| Figure refers to | 2016 |
| Last checked | |
| Unit | bacteria |
Numbers move. If this one has drifted from its source, let us know.
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