Are humans more bacteria than human cells?

The commonly repeated claim that “you have 10 times more bacterial cells than human cells” was published in a 1972 paper by Thomas Luckey and has been repeated in textbooks, science museum walls, and TED talks ever since. It is also wrong by roughly an order of magnitude.

The corrected estimate, published by Sender, Fuchs, and Milo in PLOS Biology in 2016, puts the ratio at about 1.3 bacterial cells per human cell — basically a 1:1 match. The 10:1 number came from a rough guess about gut bacteria that was never updated for the size of the other organs. For a 70-kilogram adult man, that works out to roughly 38 trillion bacteria and 30 trillion human cells, and a similar ratio for women and children. A follow-up 2018 re-analysis refined the figure further but kept the headline finding: bacterial cells are roughly equal to human cells, not ten times more numerous.

So the “you are mostly bacteria” framing is overstated, but the underlying intuition — that microbes are a much bigger part of you than most people realize — is still right. It just isn’t about cell count; it’s about mass and chemistry. The bacteria in your gut together weigh about 0.2 kg (about 7 ounces), comparable to your brain. And they collectively encode about 100 times more unique genes than your own genome. That genetic diversity is what lets them run metabolic pathways your body cannot: fermenting fiber into short-chain fatty acids like butyrate, synthesizing vitamin K and several B vitamins, breaking down polyphenols from coffee and tea, and producing neurotransmitters that influence mood and appetite.

The NIH’s Human Microbiome Project (HMP), launched in 2007, mapped the bacterial communities on the skin, gut, mouth, nose, and urogenital tract across hundreds of healthy adults. The Knight Lab’s American Gut Project extended that with thousands of citizen samples. The consistent finding is that each person’s microbiome is distinct — like a fingerprint — but the broad functional categories (the kinds of jobs the bacteria do) are stable across healthy adults. When those categories break down, the downstream effects include inflammatory bowel disease, type 2 diabetes, certain autoimmune conditions, and some treatment-resistant depression.

If you’re trying to support a healthy gut microbiome, the practical levers are not “kill bad bacteria with probiotics” — that framing is too simple. The lever that has the strongest evidence is diversity of plant fiber: 30+ different plants per week correlates with higher microbial diversity than any probiotic supplement. Exercise, sleep, and limiting unnecessary antibiotics also matter. For related questions, see can a high-sodium diet affect the gut microbiome and what is the difference between prebiotics, probiotics, and postbiotics for gut health.