There Are More Bacterial Cells Than Human Cells

When you think of your body, you probably picture a single, unified organism. In reality, you are a walking, talking ecosystem teeming with microbial life. The most recent and rigorous scientific estimates suggest that for every human cell making up your tissues and organs, there is at least one bacterial cell living on or inside you. This brings the count to a near 1-to-1 ratio, with a slight edge for our microscopic residents. This community of trillions of organisms, known as the human microbiome, is a fundamental part of what makes us who we are.

For decades, you may have heard a more dramatic claim that bacteria outnumber our own cells by a staggering 10-to-1. This popular figure originated from a rough estimate made in the 1970s and became widely cited. However, a 2016 study revisited the calculation with more precise data, counting the cells in an "average man" and providing the updated, more balanced figure. This revision highlights how scientific understanding evolves as our tools for measurement and analysis improve.

Despite their vast numbers, these bacteria don't add much to our weight on the scale. The reason is a simple matter of size. Bacterial cells are significantly smaller and simpler than human cells. While they may slightly outnumber our native cells, their collective mass is almost negligible. All 38 trillion of those bacteria combined only account for about two to six pounds of an adult's body weight, or roughly 1-3% of their total mass, proving that a population's size doesn't always reflect its physical impact.