# How many cells are there in the human body?

The other day, Amanda, who is currently teaching AP Biology, noted that among the various sources she had at hand, including a couple of textbooks, the number of cells that make up human body seemed to range from about five trillion to fifty trillion with a scattering of numbers in between. It is not clear why this number matters but I suppose if we want to impress students with the smallness of cells and the complexity of life it is worth pointing out, and if it is worth pointing out it might be worth getting it right. So, how many are there?

I believe the correct answer is in the upper end of the range Amanda cited, and here’s why.

According to various sources, the following is more or less true:

Adult people (for our present purposes) weigh between 60 and 90 kg. (I’m ignoring small populations here because this is mainly for American Audiences.) The amount of bone in a body … the non-cellular part … is between 14 and 20 percent. The amount of blood that is not cellular (i.e, that is water) is about 5% of the total body mass. The mass of a typical body cell is about one gram times ten to the negative nine, or one nanogram.

… do the math …

I get about 46 to 68 trillion.

Bigger people (using these weight ranges) would have a larger proportion of lean mass in bone, and if some off that extra mass in the range of human weights is increased fat percentage, then some of that mass is accounted for by either more fat cells or enlarged fat cells. For these and other reasons, as mass goes up the rate of additional cells goes down, so the higher end of that range is probably an exaggeration. There are other things in the body that need to be subtracted as well, including connective tissue that has very few cells in it, bacteria welcome and unwelcome alike, etc. etc.

Which brings us to a comfortable estimate of “about 50 trillion, give or take a few trillion.”

Is that satisfactory?

You can get most of the base numbers here, of course.

1. #1 Michael Fisher
November 28, 2011

You’re underestimating by a factor of around 20 according to THIS Wiki:

Gut flora consists of micro-organism that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with micro-biota and micro-flora.

The human body, consisting of about 100 trillion cells, carries about ten times as many micro-organisms in the intestines. The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a “forgotten” organ. It is estimated that these gut flora have around 100 times as many genes in aggregate as there are in the human genome.

I assume from this that the average gut flora cell must weigh only 2%-5% of an average ‘human DNA’ cell. Is that plausible?

I can’t quite make my mind up if it’s fair to exclude gut flora [& skin bacteria etc.?] from the calculation, but my instinct is that one should not ~ it’s almost as essential for our functioning as air.

2. #2 bks
November 28, 2011

About 90% of the cells (by number, not mass) in your body are bacteria. And yes, they are part of what makes you, you.

–bks

p.s. BTW Greg, did you mention the passing of Lynn Margulis last week?

November 28, 2011

bks, it went without saying that we are referring to the cells of the human body, i.e., mainly somatic sells. The relationship to bacteria may be important, even vital, but we don’t count them as somatic cells. They are different. Unless they are Endosymbionts of course!

Yes, I did mention LM’s passing in a blog post a while back.

November 28, 2011

Michael: OK, air is essential. Therefore the mass of a human is the weight the scale tells you plus the atmosphere? Oh wait, the O2 in the air comes from plants and algae and stuff. So that must be included as well!

I heartily disagree! There are numerous vital, essential links between an organism and other things … other organisms included. That does not make those other organisms that organism. The gut flora is a “forgotten organ” only metaphorically. We require our gut flora to survive. We require the organisms that convert CO2 to O2 to survive. We require the primary consumers that convert sun and water and CO2 to carbohydrates and we require the intermediate consumers that convert carbohydrates into various other things we need and concentrate minerals etc.

I have absolutely no difficulty counting the number of human cells by only counting the (mainly) somatic cells that (mainly) come from division from a fertilized egg. That is a simple biological question. There is no question as to where to draw that line. Counting only somatic cells when asking how many body cells they are (body = soma) is not dissing the gut flora or the other bacteria associated with the human body.

Regarding your source, it is interesting that they happen to say 100 trillion cells. This is why I wrote this blog post. We’ve seen this huge range of numbers.

Do you have the data and math to back up the number 100 trillion? I’ll be happy to revise. But we’re not adding in the gut flora. They will have to make due with a different, but still important, blog post at some other time!

5. #5 Lorax
November 29, 2011

I disagree with you Greg. The idea that your microflora is trivially a part of you, as I interpret your algae example, disregards a decade+ of recent research. Besides playing a role in vitamin production and nutrient acquisition, which you note, the gut microflora plays an important role in regulating the immune system. I think the research is showing a much stronger interaction between your flora and your overall health than is generally appreciated. I agree your flora is not the same as your somatic (and germ line) cells, but they play a more direct role in you being you than algae.

Question, how much shit is in the gut (by weight) and does this affect on the calculation?

November 29, 2011

Lorax, I certainly do not think microflaura are trivially part of you. There is nothing trivial about it, and I never used the word trivia. The aglae I was talking about btw was the algae in the sea. I really, really don’t want to count that as part of the human body. I’m having enough trouble loosing weight as it is!

The count of somatic cells is the count of somatic cells. Not of other cells or other things.

How many kilos do your bones weigh? What if we weighted the bones and found out they were 10 kilos. Now, did we do that wrong? Bones are nothing without muscle, and in fact, some bones only form if there is muscle and tendon and other connective tissue in a certain location (the sigmoid bones). Therefore, they are non trivially integrated, almost symbiotic with the muscles. So, to answer the question “how much bone is there” we must add in the weight for the muscle or we are doing it wrong.

Another way to look at this: How much gut flora is there in the human body? If you can’t count the cells in the human body without counting the gut flora cells as part of the body, is the reverse not true?

As noted, there is other “stuff” in the body. Bolus, chyme and feces would be “stuff”

7. #7 Jim Thomerson
November 29, 2011

It is clear from context and methodology that Greg was counting the human cells in a human and not including those of other species. That there are large numbers of bacteria associated with the human body is interesting and important, but not relevant to Greg’s original post. It is, in fact, reasonable to treat gut contents as topologically external to the body proper. They are in a tube open to the outside at both ends, after all. One often sees the analogy between the gut and the hole in a donut. (At least this was so back in the 20th century.)

8. #8 T
November 29, 2011

I’m with Greg and Jim, intestinal microflora shouldn’t be counted as human cells anymore than bacteria on the surface of our skin should be counted. If you think about it, other than the cardiac and anal sphincters sealing it off, the digestive tract is exposed to the environment, the “tube within a tube” body plan.

9. #9 BaisBlackfingers
November 30, 2011

I’m not sure about the 1 ng estimate. Larger cells contribute disproportionate amounts of mass and may skew this. Skeletal muscle in particular constitutes large fractions of our total body mass but these cells can run the whole length of a muscle, containing hundreds or thousands of nuclei. More reasonably sized muscle stem cells and fibroblasts also exist in this tissue, but do not make up the bulk of the mass.

November 30, 2011

BaisBlackfingers: good point, but you need to supply some numbers!

11. #11 BaisBlackfingers
November 30, 2011

Greg- I don’t think anybody has real numbers for the weight of an average muscle fiber- there is alot of heterogeneity based on length from muscle to muscle. I have spotted a few numbers in the 1 ug per mm of fiber length range (ranging from .7 to 1.5) for dry weight, so a little over 3ug/mm hydrated I would guess. I don’t know the mean length (parallel to pennation) of a muscle in the human body (any human anatomists care to chime in?) but with those numbers you could do it.

12. #12 Mike Lewinski
December 6, 2011

But we’re not adding in the gut flora. They will have to make due with a different, but still important, blog post at some other time!

I’m looking forward to that post.

A couple months ago Popular Science wrote an article saying that our bacteria collectively weigh 3-5 pounds.

This Google Answer on Bacteria and Human Body Weight widens the range to 2-9 pounds and cites a wide variety of sources.

I’m just as curious about cell count as weight, and particularly interested about the ratio of somatic to non-somatic cells.

It would also be interesting to know more about the ratio of DNA between human and bacterial base pairs. In her TED talk on quorum sensing Bonnie Bassler says it is 1:100. She also says that the somatic cell count is about 1 trillion.

In Lives of a Cell Lewis Thomas pondered a similar question:

My mitochondria comprise a very large proportion of me. I cannot do the calculation, but I sup- pose there is almost as much of them in sheer dry bulk as there is the rest of me.

13. #13 Abel
March 28, 2012

There’s to many things to substitute into this equation to give any form of “accurate” estimate, however, that’s just my honest opinion.

14. #14 Shmoo
May 23, 2012

Thanks! That’s really interesting. I was wondering how scientist calculated the number of cells the average human body contained and I thought they couldn’t possibly count it by hand!

15. #15 persons
December 18, 2012