Saturday, June 29, 2013

A Matter of Scale

This morning I began wondering how a bacterium living on a human being would compare, in scale, to a human being living on the Earth. Before I present you with the actual comparisons, first we need some dry facts:
  • An average cell of E. coli is 2 µm long, 0.5 µm in circumference, and 0.6 µm3 in volume.[1][2]
  • An average adult human being is 1.7 m tall [3], 96 cm in circumference [4], and 0.07 m3 in volume.[5]
  • The Earth is (on average) 12,742 km in diameter; 40,075 km in circumference; and 1.08 trillion km3 in volume.[6]
And for fun:
  • The moon is (on average) 1737 km in diameter; 10,921 km in circumference; and 22 billion km3 in volume.[7]
Now for the calculations:

If we blew up an E. coli cell to the size of a human (for convenience, let's round a human being to 2 m tall), it would be 50 cm in diameter and 0.6 m3 in volume. (The differences in volumes between a human and a human-sized E. coli are due either to an inaccurate calculation for human volume, the fact that E. coli cells are much closer to true cylinders than we are, and that I gave the E. coli and extra 0.3 m.)

If we blew up a human being at the same time, then the human-sized E. coli would be living in a human being that was 1700 km tall, 960 km in diameter, and 70 million km3 in volume. That's about ⅛th of the diameter of the Earth and half the diameter of the moon. If such a person were standing on the surface of the Earth, the International Space Station would be flying past their knees.

And now you know.


[1] See coli#Biology and biochemistry.

[2] E. coli technically lives in your digestive tract, which, in a sense, is still on the outside of you (see note [#] below). I chose it because it's a well-known bacterium and because of its shape it is more useful for comparison to a human being than the common skin bacterium, Staphylococcus aureus, which is spherical.

[3] See

[4] Ibid.

[5] See is the average volume space of the human body.

[6] See

[7] See

[#] Your digestive tract is essentially a long tube that goes all the way through you, kind of like the hole in a doughnut. Bacteria growing in the hole of the doughnut are still on the outside of the doughnut. Likewise the bacteria growing in your digestive tract are outside of you. There aren't (or shouldn't be) any bacteria growing on your actual insides, otherwise they can cause disease.

Image attributions:

Scanning Electrom Micrograph of Escherichia coli is by employees of the CDC (Evangeline Sowers and Janice Carr), an entity of the United States federal government. As such, the image exists in the public domain. It is available at coli (SEM).jpg.

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