How Long Would It Take Bacteria to Eat the Earth?

A trillion bacteria weigh one gram, 500 million bacteria fit in a grain of sand, and other answers from order-of-magnitude thinking.

Niko McCarty
14 min readAug 14, 2020


An artist’s rendering of microbes. Photo by Elena Mozhvilo on Unsplash.

When I first heard that bacteria in the body outnumber human cells ten-to-one, eight years ago, I thought, “Now that is truly terrifying, to be mostly bacterial, a container for microbial soup.” The thought is morbid, but I can explain its appeal.

The notion that bacterial cells outnumber human ones is intriguing because I, like others, envision bacteria as microscopic, impotent creatures that only exert their influence when causing some terrible disease. The idea that people are “mostly bacterial” removes the impotent adjective from these creatures — bacteria are interesting because they are me. In the titular words of Ed Yong, “I Contain Multitudes”.

Unfortunately, it turns out that the ten-to-one thing is not true. Bacteria and human cells probably inhabit our bodies at a one-to-one ratio. But I still think of bacteria as mighty beings, packed into a small frame.

Our skin is teeming with microbes, as depicted in this schematic from the National Human Genome Research Institute. Image from Flickr.

Last year, a class at the California Institute of Technology further solidified, for me, the dynamism of the microbial world. The course, called Physical Biology of the Cell, was taught by an enigmatic professor named Rob Phillips. As a graduate student in the Phillips lab, I considered it my solemn duty to enroll.

Expecting a semester filled with esoteric math equations, I was a bit surprised when the first two weeks of the course were devoted entirely to arithmetic; what the syllabus called order-of-magnitude thinking.

Order-of-magnitude thinking is a way to estimate mathematical values using nothing but your brain, some paper, and a pen. Basically, if you were stuck on a desert island, with nothing but a stick, and rescue was assured only if you could answer a series of questions, you could very nearly answer questions like how many red blood cells are in the human body? and how many viruses are in one drop of ocean water?



Niko McCarty

Science journalism at NYU. Previously Caltech, Imperial College. #SynBio newsletter: Web: