Microorganisms constitute the bulk of all the biomass on Earth. I weighed myself yesterday, and wondered how much less I would weigh if I were completely free of bacteria and microbes, inside and out.

Approximately how much weight and volume do microbes occupy within the average human body? How were these values obtained?


3 Answers 3


@AlanBoyd's calculations are reasonable, I think the estimate is off though. The human microbome includes other bacteria which are not necessarily E.coli equivalent.

The human microbome projects give estimates that microbes are 1-3% total body mass. i.e. several pounds of bacteria.

The GI tract alone has most of the microbome mass - faeces is ~60% intestinal flora/fauna by dry weight, which for many adults alone must be hundreds of grams at any given moment.


The bacterial flora weighs approximately 90 g and, assuming bacterial cell density is approximately 1 g ml-1, occupies a volume of 90 ml.

The human body has 1013 human cells and hosts 9x1013 bacterial cells.

One E. coli cell has a mass of 0.95×10−15 kg (wet weight).

Assuming that E. coli is typical with respect to mass of human microbial flora:

mass of bacterial cells in one human body = (0.95×10−15 * 9x1013) kg = 0.0855 kg = 86 g

  • $\begingroup$ Just human intestines have over a kilogram of bacteria! A good enema will leave you 2kg lighter and that's mostly bacteria. $\endgroup$ Dec 3, 2012 at 21:09
  • $\begingroup$ The number I read where more like 1-2 orders of magnitude more bacterial cells than cells in our body. $\endgroup$
    – Chris
    Dec 20, 2013 at 9:56

According to this source, it's about 2 kg:

Our gut microbiota contains tens of trillions of microorganisms, including at least 1000 different species of known bacteria with more than 3 million genes (150 times more than human genes). Microbiota can, in total, weigh up to 2 kg. One third of our gut microbiota is common to most people, while two thirds are specific to each one of us. In other words, the microbiota in your intestine is like an individual identity card.

Other source, confirm it:

Le chiffre a de quoi surprendre. Il s’explique par le fait que les bactéries sont en général beaucoup plus petites que les cellules de notre propre organisme. Il y en a donc une quantité énorme dans un volume restreint; enfin sachez quand même que nous portons en moyenne 1 à 2 kilos de bactéries, pour la plupart localisées dans notre colon. Il semblerait d’ailleurs que le colon humain soit l’un des écosystèmes les plus denses que l’on connaisse.


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