Let's Get Small - Experiment

hairJust how small is a microbe? If you happen to read a report written by a microbiologist, you might learn that a poliovirus is 30 nanometers in diameter and that an E. coli cell is 3 micrometers long. But does that mean anything to you? Scientists use these measurement terms because microbes are so small, they cannot be measured using the more familiar inches or millimeters. (A micrometer is a thousand times smaller than a millimeter and a nanometer is one million times smaller than a millimeter.)

If you read What is a Microbe, you came across an illustration of microbial size that compared different microbes to common features of a baseball park. Such comparisons can help you put into perspective how small microbes are compared to, say, a human cell using objects whose size is more familiar to you. That’s what this activity is all about. You’ll compare the sizes of different microbes to the size of a very familiar item—a human hair.

Note: This activity will take about 1 hour.

You’ll Need:


  • meter stick or ruler
  • magnifying glass
  • access to an empty parking lot, carport, patio, gymnasium, field or other large, open space.
  • depending on the type of space you have, you’ll need either a box of sidewalk chalk (parking lot or patio), colored string (field or other grassy area), or colored tape (gymnasium)
  • a hair pulled from your head or taken from your brush or comb
  • Print-out of Microbe Reference Chart

Note: If you do this activity in a parking lot, choose a day when that lot is not being used. You should definitely have at least one friend with you if not a parent or other adult. At least one person should keep a lookout for approaching cars at all times. Just be careful!

What To Do:

adenovirus1. Examine your single hair both with and without the magnifying glass. Note how thin it is if you hold it at one end and look down at the tip. Depending on how thick your hair is, you may just barely be able to see it without the magnifying glass. The average human hair is 0.1 millimeters wide, or one-tenth of one millimeter wide. That’s pretty tiny.

2. In your open space, use your meter stick to measure out an area that is 10 meters long. Mark off this 10-meter area with your chalk, string or tape, whatever works best for your chosen space. This 10-meter area will represent the width of your hair, that tiny width that you were barely able to see without a magnifying glass.

3. Choose one of the microbes from the first column of the Microbe Reference Chart. Think about how its size might compare to the width of your hair. Using the model size listed in column 4 of the Chart and your meter stick to measure, draw a picture of this microbe (or mark off its length and width with string or tape) somewhere beside the 10-meter area you already marked off. Once again, hold up your hair and see how tiny its width is and compare the actual size of the hair with the actual size (column 2) of the microbe.

paramecium4. Repeat step 3 with additional microbes listed in the Reference Chart. You can do as many as you like. Use different colored chalk, tape or string for each microbe so you can remember which markings refer to which microbe.

5. When you’re finished drawing as many microbes as you’d like, compare the sizes and shapes of the microbes you've drawn. You have just created a reference model for microbial size.

6. Clean up your remaining pieces of chalk and remove tape or string when you’re done.


  1. Why do scientists need to use models? Why can’t they just look under a microscope to see what they need to see? Answer
  2. Which kind of microbe is the smallest, a virus, bacterium or protozoan? Which is the biggest? Answer
  3. According to the Reference Chart, an E. coli bacterium is 2 micrometers long. Assuming that it's 1.25 micrometers wide, could you determine how many E. coli could fit on the tip of one of your hairs? Answer

This experiment is based on an activity developed by the National Association of Biology Teachers.


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