National Geographic

How Many Cells Are In Your Body?

A simple question deserves a simple answer. How many cells are in your body?

Unfortunately, your cells can’t fill out census forms, so they can’t tell you themselves. And while it’s easy enough to look through a microscope and count off certain types of cells, this method isn’t practical either. Some types of cells are easy to spot, while others–such as tangled neurons–weave themselves up into obscurity. Even if you could count ten cells each second, it would take you tens of thousands of years to finish counting. Plus, there would be certain logistical problems you’d encounter along the way to counting all the cells in your body–for example, chopping your own body up into tiny patches for microscopic viewing.

For now, the best we can hope for is a study published recenty in Annals of Human Biology, entitled, with admirable clarity, “An Estimation of the Number of Cells in the Human Body.”

The authors–a team of scientists from Italy, Greece, and Spain–admit that they’re hardly the first people to tackle this question. They looked back over scientific journals and books from the past couple centuries and found many estimates. But those estimates sprawled over a huge range, from 5 billion to 200 million trillion cells. And practically none of scientists who offered those numbers provided  an explanation for how they came up with them. Clearly, this is a subject ripe for research.

If scientists can’t count all the cells in a human body, how can they estimate it? The mean weight of a cell is 1 nanogram. For an adult man weighing 70 kilograms, simple arithmetic would lead us to conclude that that man has 70 trillion cells.

On the other hand, it’s also possible to do this calculation based on the volume of cells. The mean volume of a mammal cell is estimated to be 4 billionths of a cubic centimeter. (To get a sense of that size, check out The Scale of the Universe.) Based on an adult man’s typical volume, you might conclude that the human body contains 15 trillion cells.

So if you pick volume or weight, you get drastically different numbers. Making matters worse, our bodies are not packed with cells in a uniform way, like a jar full of jellybeans. Cells come in different sizes, and they grow in different densities. Look at a beaker of blood, for example, and you’ll find that the red blood cells are packed tight. If you used their density to estimate the cells in a human body, you’d come to a staggering 724 trillion cells. Skin cells, on the other hand, are so sparse that they’d give you a paltry estimate of 35 billion cells.

So the author of the new paper set out to estimate the number of cells in the body the hard way, breaking it down by organs and cell types. (They didn’t try counting up all the microbes that also call our body home, sticking only to human cells.) They’ve scoured the scientific literature for details on the volume and density of cells in gallbladders, knee joints, intestines, bone marrow, and many other tissues. They then came up with estimates for the total number of each kind of cell. They estimate, for example, that we have 50 billion fat cells and 2 billion heart muscle cells.

Adding up all their numbers, the scientists came up with…drumroll…37.2 trillion cells.

This is not a final number, but it’s a very good start. While it’s true that people may vary in size–and thus vary in their number of cells–adult humans don’t vary by orders of magnitude except in the movies. The scientists declare with great confidence that the common estimate of a trillion cells in the human body is wrong. But they see their estimate as an opportunity for a collaboration–perhaps through an online database assembled by many experts on many different body parts–to zero in on a better estimate.

Curiosity is justification enough to ponder how many cells the human body contains, but there can also be scientific benefits to pinning down the number too. Scientists are learning about the human body by building sophisticated computer models of lungs and hearts and other organs. If these models have ten times too many cells as real organs do, their results may veer wildly off the mark.

The number of cells in an organ also has bearing on some medical conditions. The authors of the new study find that a healthy liver has 240 billion cells in it, for example, but some studies on cirrhosis have found the disease organ have as few as 172 billion.

Perhaps most importantly, the very fact that some 34 trillion cells can cooperate for decades, giving rise to a single human body instead of a chaotic war of selfish microbes, is amazing. The evolution of even a basic level of multicellularity is remarkable enough. But our ancestors went way beyond a simple sponge-like anatomy, evolving a vast collective made of many different types. To understand that collective on a deep level, we need to know how big it really is.

There are 46 Comments. Add Yours.

  1. David Sanders
    October 23, 2013

    Good article. Unfortunately it misses an opportunity to point out that the 37 trillion cell number is an order of magnitude too low. It doesn’t count the microbes that are actually integral to the human body.

    [CZ: The study I'm writing about didn't estimate the size of the microbiome. It's only looking at human cells. I've added a note to make that clear.]

  2. David Krogh
    October 23, 2013

    So, where does this leave the number of microbiome cells in the human body vs. human cells? The most common previous estimate of human cells was 1 trillion. With that number, it was easy to say that the microbiome cells outnumbered our own, as the number of microbiome cells was commonly thought to be 10 trillion. If this latter estimate is still believed to be true, then does the recent estimate of the number of human cells overturn the truism that most of the cells in the human body aren’t human?

  3. Jack Gaynor
    October 23, 2013

    For those interested in a class exercise that will allow your students to arrive at their own value, I wrote a lab protocol called the “Jellybeans in the jar dilemma” that you can use. I’ve done this with middle school through college level classes and it’s never failed to generate excitement. http://www.pearsonhighered.com/strategies/assets/pdf/greatideas/GreatIdeas_Biology_1.pdf

  4. Kathy K.
    October 23, 2013

    It is a hard concept to visualize because #’s don’t equate with volumes. Human cells can take up a greater volume even though the microbiome has many more #’s of cells. I’d like to see not only a comparison of numbers but also a comparison of volumes.

    This is a good discussion for Mole Day (6.02 am- 6.02 pm)

  5. Physicalist
    October 23, 2013

    Thanks for a fascinating answer to a question I’ve often wondered about.

    But the philosopher in me cries for consistency. You say, “37.2 trillion cells” and then in the last paragraph you mention “some 34 trillion cells.” Don’t you want these numbers to match?

  6. Jim Everett
    October 23, 2013

    I have no problem comparing estimates from 1 to 100; 37 sounds not sounds like a reasonable compromise. It’s the trillion I have a problem with imagining. For instance, if every cell has a complete copy of DNA (about 6 feet in length, stretched end to end), then just that part of our bodies would combine to a length of roughly 222 trillion feet (on the back of my envelope). That’s somewhat farther than Pluto is from the Sun. It’s not that I disbelieve this. But how do I weave these huge numbers, big or small, into my sensibilty?

  7. Amit
    October 23, 2013

    Nice article.

    I remember reading that the number of microbes in the human gut are higher than this number?! Over 100 Trillion? ‏http://en.wikipedia.org/wiki/Gut_flora

  8. Daniel Noesgaard
    October 24, 2013

    The “all the microbes that also call our body home” link is broken…

    [CZ: Thanks. Link fixed.]

  9. John Adam
    October 24, 2013

    On p.59 of ‘Guesstimation: Solving the World’s Problems on the Back of a Cocktail Napkin’ (http://www.pup.princeton.edu/titles/8625.html) there is a back-of-the-envelope estimateof this quantity yielding: 100 trillion. :-)

  10. Dredd
    October 25, 2013

    Incredible post. Thanks for mentioning that the microbial (including viruses) world was not included in the count.

    May I add that each cell has over a hundred thousand machines within it, including DNA, which is a molecular machine that is not alive. DNA is composed of molecules that are not alive which are composed of atoms that are also not alive.

    In actual fact, our cells are a type of cyborg (Putting a Face on Machine Mutation).

    http://ecocosmology.blogspot.com/2013/10/putting-face-on-machine-mutation-4.html.

  11. SocraticGadfly
    October 25, 2013

    Good comments on the microbiome issue. On the weight ratio, too, will that change from the 90:10 traditional estimate of human cells by weight vs. microbiome?

  12. Maxim
    December 2, 2013

    I have confronted the number from the link of the study and it is 37 200 000 000 000. And it would be 37 BILLIONS (and not trillions). Isn’t it?

    [CZ: No. Perhaps you are accustomed to the British use of "billion."]

  13. Maxim
    December 4, 2013

    And if I just say “37 thousands of milliards”. Wouldn’t it be correctly?

    Thanks

  14. Timothy Bedard
    December 18, 2013

    I’m not sure if I did these calculations wrong but with the amount of mistakes DNA polymerase makes, and with the number of cells in our body 1,850,000,000,000 cells in our boys create mutated proteins (I have accounted for the fact that only 1.5% of DNA codes for proteins, the one thing I didn’t account for is the fact that most mutations don’t change the proteins at all.) If anybody has another number that would be great.

  15. Rick Espejo
    December 22, 2013

    According to Dredds comment, If atoms and DNA microbes are not alive, why do they die? DNA is a theorized a machine yet it has to re-create the entire human with-in the very first mother cell, which means every cell mitosis isn’t the same but ordained chronologically to complete the stages of cell life but before life begins as a baby, the sperm cell has its own mind to find the egg by competition. I believe all Cell nucleuses have their own minds via preordained DNA codes due to competition. The strangest mystery is why 50 trillion cells die instantaneously when the human dies.

  16. Paul Jacob
    January 9, 2014

    The linked article “results” statement calls for 3.72 X 10(13). That is far more than 37 trillion. It’s closer to 37 billion trillion trillion.

    • Timothy Bedard
      January 11, 2014

      @Paul Jacob
      3.72×10^13=372×10^11, or 372 with 11 zeros. 37,200,000,000,000. That is 37.2 trillion.

  17. Elizabeth berryhill
    January 25, 2014

    How many cells do we have when we’re in old age or about to pass what happen to our cells then?

  18. Rafael H.
    January 26, 2014

    Do you really know? Or were you taught what you know?
    Do you believe or do you know?
    Is your faith blind?
    Or is it bright as the sun? – Killah Priest

  19. kenneth moss bilal
    January 29, 2014

    what i find astonding, other than the fantastic number of cells, is the fact that all are similar but not one is exactly alike. all are unique. something like people

  20. Adam
    February 4, 2014

    The commentor who called DNA and similar molecular machines “not alive” seems to have fallen into the Trap of Vitalism – that “life” is definable as an essence separable from its components. DNA and kin are alive by being part of a living cell’s processes

  21. Manmohan Singh
    February 10, 2014

    Excellent article; substantive and very clear. Please let’s have more from the author. 3 questions: what are the organs/body parts with the highest numbers of cells; ie. how do they rank? Is there any systemtic association between the number of cells and the complexity of task by the organ, incidence of disease etc; and is the number of cells markedly different across different age groups?

  22. UKPETU OILOYEN
    February 15, 2014

    How many cells die a day?

  23. Jule Koch
    February 20, 2014

    And where did all those cells come from and how did they all come together and function? 0 + 0 = everything in the universe?

    “The fool says in his heart, ‘There is no God.’
    For since the creation of the world God’s invisible qualities–his eternal power and divine nature–have been clearly seen, being understood from what has been made, so that people are without excuse.
    Although they claimed to be wise, they became fools. They exchanged the truth about God for a lie, and worshiped and served created things rather than the Creator”

  24. prakash
    February 22, 2014

    could any one say me that how many blood cells are there for healthy human beings

  25. Fernando Saravi
    March 7, 2014

    Prakash: This is an easy one. Each microliter of blood has on average 5 million red blood cells, 7 thousand white blood cells and 300 thousand platelets. Average total blood volume is 5 L. Thus there are about 25.3 trillion (American usage) or 2.53 x 10exp13 cells in healthy human beings (this only includes circulating blood cells).

  26. nan
    March 11, 2014

    6th grade teacher: When we die do we have about the same # of cells as when we die? How does our physical growth factor in?

  27. nanegan
    March 11, 2014

    Do we have about the same # of body cells at birth and death? Explain physical growth. 6th grade teacher

  28. Douglas Lietzau
    March 20, 2014

    Would you please be so kind as to provide a clear definition for a
    so-called human cell? What exactly is a human cell.
    This has been haunting me for sometime.
    Thank you.
    Sincerely,
    Doug

  29. Denis Gilmore
    March 29, 2014

    A good article, thanks, but one thing it highlights to me is the silliness of saying, as you do, that one body made up of some 3 or 4 x 10(13) cells working in complete harmony, to form one single being, evolved. The dna in each of those cells contains not only a digitally encoded design for that particular cells construction & function, but that for the whole body, as it moves through all its stages of develoment. If the universe is approx 14 billion yrs old, and the earth less than 1 billion (10(12)) , that is a huge amount of developnent per year. I believe there to be tens of megabytes of hugely clever and compact code per dna, so some intelligence had to imagine the complete design, then encode it. I think the average human prgrammer is only capable of tens of bytes of code per day, so how many days does it take for random chance to produce a million (mega, i.e. 10(6)) times that. Anyway, a biochemist estimated that, starting with the basic elements and joining them, there is only 1 chance in 10(125) that one of the protiens required to sustain life would be formed, and approximately 250 different such proteins are required, and that before the dna code can be written, and none of those chemicals and codes give life they are just required to sustain and build it, and are still there when life departs from the human body, though start breaking down (decaying) as soon as it does.
    No wonder you said it was “remarkable”, some would say that it is actually a mathematically ridiculous (prepostuous) supposition.

  30. Ochiamu Aloysius
    April 3, 2014

    In Nigeria here I am, we are told that there are 200 different types of cells and about 100trillion cell.So does it mean that there no universal compendium of the number of cell in human?

  31. arthur
    April 8, 2014

    Any cell commonly found within an organism commonly known as human ?

  32. Señor Pedo
    April 10, 2014

    Thank for giving a simple awnser

  33. Isnejcjdn
    April 10, 2014

    Cool

  34. El Pedo 2
    April 10, 2014

    I agree with El Pedo

  35. Cathie
    April 14, 2014

    Perhaps you can help me with a question that has been bothering me: when cells split, that presumably means dividing up the number of constituent atoms too?? Does that mean that each subsequent cell split results in cells that are less dense (i.e. have fewer atoms) and that the original cell contained all the atoms available? Or do cells absorb atoms from other substances, so that ultimately a cell split is something dictated by processes taking place at the atomic level?

  36. Maggie
    April 21, 2014

    again….

    …Amazing Creation!..

    Pride, ego and vane glory comes first while we are alive….but the moment of truth (for all) comes at the time of death.

  37. Rebeca
    April 30, 2014

    AWESOME!!!!

  38. Phil Rock
    May 31, 2014

    I have one question about the total volume of blood cells. It seems like the quantity of blood in the average human would comprise about 70% of the total number of cells. Is this reasonable?

  39. Shankar Bisanal
    June 18, 2014

    How much small size of the cells in human body?

  40. Mike Lewinski
    July 24, 2014

    Carl if you’re still following this thread could you look into the question of the number of red blood cells asked above?

    As you know mature red blood cells do not contain DNA.

    I’m attempting to estimate the number of mutations caused by normal cell metabolism in an average human body each day. There’s a fairly well established figure of 20,000 mutations per cell, per day:

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2664861/

    Without subtracting the red blood cells, 20,000 * 37.2 trillion = 744 quadrillion mutations that occur and are repaired in the average human body each and every day.

  41. Mike Lewinski
    July 24, 2014

    Via pubmed references I did finally find an answer even though the full text of the paper is behind a paywall. It appears there are 2.63 x 10^13 erythrocytes. The table listing numbers for each organ is available as PDF download from this link:

    http://bionumbers.hms.harvard.edu/bionumber.aspx?&id=109708&ver=6

    So if my math isn’t horribly wrong, that amounts to 10.9 trillion cells containing DNA, which in turn equals 218 quadrillion mutations per day in the average human.

  42. elina
    August 3, 2014

    awesome!!! and thanks for giving me a simple answer

  43. Nirmal
    September 14, 2014

    It is noted in Guyton & Hall’s physiology text book that there are about 25 trillion Red Blood Cells in human body and a total of 100 trillion cells in the whole human body.So the number 37.2 trillion cells in total human body, isn’t it a bit controversial?

  44. Tasarran
    November 8, 2014

    This would be what is known as science, @Nirmal… Not sure how Guyton and Hall arrived at their numbers, but given the roundness of those numbers, it seems like they did some of the math that was done at the beginning of this article, then rounded up. In any event, I seriously doubt they went to anything like the rigor described in this article…
    Better, more organized, methodical approaches get better results, the old answers get changed, and our picture of the universe gets a little more accurate.
    Also, it’s kind of meaningless to quibble over this number anyway, the best we will ever have is an average.

  45. varsha
    December 10, 2014

    Thanks for easily understand , It so wonderfull …
    Thank you so much

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