Your Body Has 38 Trillion Bacteria (Not 100 Trillion)

For decades, science popularizers loved to drop this jaw-dropping stat: your body contains 100 trillion bacteria, outnumbering your human cells 10 to 1. It showed up in textbooks, TED talks, and probably your high school biology class. There's just one problem—it's wrong.

In 2016, researchers finally did the detailed math and discovered the real number: about 38 trillion bacterial cells living alongside roughly 30 trillion human cells. That's a ratio of about 1.3 to 1, not 10 to 1. Still impressive, but nowhere near the old claim.

The Back-of-the-Envelope Math That Fooled Everyone

The 100 trillion figure came from a 1972 estimate by biochemist Thomas Luckey. He calculated that each gram of human feces contained 100 billion bacteria, and assumed the digestive system held about one kilogram of material. Multiply those numbers and boom—100 trillion bacteria.

The problem? It was literally a "back-of-the-envelope" calculation meant as a rough reference point, not gospel truth. But once it entered the scientific literature, it took on a life of its own, getting cited and recited for over 40 years.

What Changed?

Israeli researchers took a more rigorous approach, carefully measuring bacterial density in different parts of the body and accounting for factors like body weight, colon volume, and the fact that most bacteria live in your gut, not evenly distributed throughout your body.

Their revised estimate: the average 70 kg man has about 38 trillion bacteria, with an uncertainty of about 25%. Your total bacterial mass? About 0.2 kilograms, or less than half a pound.

Still a Thriving Ecosystem

Even with the corrected numbers, your microbiome remains incredibly diverse. Scientists estimate 500 to 1,000 different bacterial species colonize your body at any given time, with the gut alone hosting up to 7,000 distinct species.

The genetic diversity is even more staggering. Those bacteria collectively carry about 2 million genes—100 times more than the roughly 20,000 genes in human DNA. Your microbiome doesn't just live in you; it fundamentally shapes your digestion, immune system, and even mood.

So while you might not have 100 trillion microscopic roommates, the 38 trillion you do have are more than enough to keep things interesting.