For decades, a popular claim held that bacteria outnumber human cells 10-to-1 in the body. A rigorous 2016 study by Sender et al., published in PLOS Biology, debunked that exaggeration with careful cell-by-cell accounting. Their revised estimate puts bacterial cells at approximately 3.8 × 10¹³ and human cells at approximately 3.0 × 10¹³ — a ratio of about 1.3:1, not 10:1.
This means bacterial cells constitute roughly 56% of all cells in the body — just over half. The dominant human cell types are red blood cells and platelets (accounting for ~90% of the human cell count), while gut bacteria make up the vast majority of microbial cells. The NIH's current understanding of the human microbiome literature corroborates this near-equal ratio.
Importantly, the 1.3:1 ratio applies to a 70 kg reference male and carries roughly 25% uncertainty, so the figure varies by body size, sex, and individual microbiome composition. By mass, bacteria contribute only about 0.2 kg — so while they nearly match human cells in number, they represent a tiny fraction of total body weight.