The short answer is that the body of an average man contains around 30 to 40 trillion cells. The long answer is that scientists do not yet know the exact number. Plus, it depends on whether or not you include the bacteria that are present in and on our bodies.
The majority of the cells in our bodies are actually red blood cells. Although they make up over 80 percent of our body in number, they constitute only around 4 percent of total body mass. This is because red blood cells only measure on average 8 micrometers in diameter, which is 10 times smaller in diameter than an average human hair.
In contrast, the average size of a fat cell is 100 micrometers. Although fat cells make up nearly 19 percent of body mass, they contribute under 0.2 percent to the total cell number.
But why is it so difficult to figure out the exact number of cells in the body?
Lack of coordinated effort
In 2013, a team of researchers from Greece, Italy, and Spain published an estimation of the number of cells in the body. They used data reported by others about individual organs and some mathematical modeling to obtain their results.
This paper put the number of cells at 37.2 trillion, plus or minus around 0.81 trillion.
Senior author Pierluigi Strippoli, an associate professor of applied biology at the University of Bologna in Italy, told Medical News Today that it was "difficult to obtain exact data for diffuse systems," such as blood vessels and nerves.
In fact, it was impossible for the team to investigate all of the organs and cell types in the body, so this number is an "initial effort," Prof. Strippoli explained.
He added that he and his team "hope that further contributions published by organ specialists will help improve the human body cell count estimation." Did other scientists take up this call?
Human and bacterial cells
They did. Senior study author Ron Milo, an associate professor at the Weizman Institute of Science in Rehovot, Israel, and colleagues published an update in 2016.
In fact, they performed two different calculations. The first one estimates the number of cells in a 100-kilogram male using an average cell volume of between 1,000 and 10,000 cubic micrometers. This gave them a "back of the envelope estimate" in a range of 30 to 40 trillion cells.
Next, they calculated the actual cell number of the five most common cell types in an average adult male, which account for 97 percent of the cells in the body. This led them to an estimate of 30 trillion cells, of which red blood cells make up 84 percent.
But human cells are not the only cells in our bodies. Although previous studies have estimated that there are 10 times as many bacteria in our bodies than human cells, Prof. Milo and colleagues revised this number to be around 38 trillion.
Interestingly, although large in number, bacteria are much smaller than human cells, and they actually make up only 200 grams of total body mass, according to Prof. Milo.
But with nearly equal numbers of cells in our bodies, one could argue that we are as much bacteria as we are human, bringing the total number up to around 70 trillion.