Jump to main content

Small, Tiny, Invisible: How Big Is a Virus?

4 reviews


Active Time
30-45 minutes
Total Project Time
30-45 minutes
Key Concepts
Microbiology, Virus
Svenja Lohner, PhD, Science Buddies

This activity was inspired by "The Science of Microbes, Comparing Sizes of Microorganisms," activity from the Center for Educational Outreach at Baylor College of Medicine

Model the Size of a Virus


Have you ever seen a real virus? Probably not—they are so tiny that it is impossible to see them with your naked eyes. But how big or small are viruses really? The coronavirus Sars-Cov-2, which causes COVID-19, is about 120 nanometers in size. One nanometer is equal to one billionth of a meter! How many of these tiny coronaviruses do you think can you line up inside a pencil dot? In this activity, you will find out by creating a scale model that allows you to compare the size of many different small objects.

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


  • Units and Conversion Table
  • Printed Scaling Chart
  • Meterstick or measuring tape
  • Yarn or string (at least 12 m long); thin yarn or yarn with a medium thickness works best.
  • Scissors
  • Paper
  • Pencil and pencil sharpener or a 1-mm mechanical pencil
  • Tape
  • Permanent marker
  • Optional: Magnifying glass
  • Optional, if you measure in two dimensions: Large space (10 m2) either outside (driveway, yard, park) or indoors (wall, floor)
  • Optional, if you measure in two dimensions: Painter's tape, additional yarn or string, or chalk
    Materials needed for the 'Small, Tiny, Invisible: How Big Is a Virus?' activity.

Prep Work

  1. Review this Units and Conversion Table to familiarize yourself with the units of measurements for length. In this activity we will be using the International Systems of Units (SI).
  2. Look at the markings of your meterstick and the measuring tape. If you do not know yet, find out what the lines on the meterstick or measuring tape mean.
    Think about:
    Which lines mark a millimeter? Which lines mark a centimeter?


Part 1

  1. Before you make a scale model of the coronavirus, you are going to take a closer look at something very small that you can still see with your naked eyes. Take the sharpened pencil or a mechanical pencil with a 1-mm lead and make a pencil dot on a sheet of paper by rotating the pencil back and forth on one spot. Look closely at the dot, either with just your eyes, or you can use a magnifying glass if you have one.
    Think about:
    How well can you see the pencil dot with your eyes? How does it look different through the magnifying glass?
  2. Use the ruler to estimate the length of the pencil dot.
    Think about:
    What is the estimated length of your pencil dot?

    A pencil dot is measured with a ruler.
  3. An average pencil dot is about 1 millimeter (mm) in size (1 mm long and 1 mm wide). If your pencil dot has a different size, try to make another pencil dot that has a size of 1 mm. Then review the Units and Conversion Table again.
    Think about:
    How many 1-mm-sized pencil dots would fit on a 1-centimeter (cm) or 1-meter (m) pencil line?
  4. Viruses and bacteria are commonly measured in nanometers (nm) and micrometers (μm). Most known viruses have a diameter between 20 and 300 nm. The size of most bacteria, on the other hand, ranges from 0.2 to 2  μm in diameter.
    Think about:
    How many micrometers fit in 1 mm? How many nanometers fit in 1 mm?
  5. Next, you will make a scale model of a specific virus, the coronavirus Sars-Cov-2, which is 120 nm in size. A scale model allows you to understand the relative size of different objects. A toy car, for example is a scale model of a real car. It looks the same as a real car, but much smaller. You will start with a one-dimensional scale model, which means that you will only consider the length of the object. In your scale model, every object will become 10,000 times larger than it is in reality.
    Think about:
    How long is the 1 mm-sized pencil dot in your scale model (in meters)?
  6. Take the yarn and measure 10 m with your measuring tape, but be sure not to pull the yarn taut, as that will distort your measurements. Cut the yarn at 10 m. If you have the space, lay it out in one stretch or in a zigzag pattern. In your scale model, the length of this string of yarn represents the length of a pencil dot. Write a label that says "pencil dot" on a piece of paper and attach it to the string with tape.
    Think about:
    Now that the pencil dot has become 10 m long, how long do you think a virus would be in your scale model?

    A 10-meter long piece of string with an attached label that says  'pencil dot'
  7. Look at the Scaling Chart. It will give you the average real-life sizes of several objects, including microorganisms such as the Escherichia coli bacterium or the coronavirus. Remember that in your scale model the length and width of every object is enlarged by 10,000 times.
  8. Model the length of each of the objects highlighted in red on the Scaling Chart. In step 6 you already cut a 10 m long (or 10,000 mm long) piece of string that represents the 1-mm pencil dot in your scale model. Next, cut a piece of string that represents the diameter (or thickness) of a human hair. Label the piece of string so you know what it represents. For your model, assume an average hair diameter of 100 μm.
    Think about:
    How long does the piece of string need to be to represent a 100 μm-thick human hair?

    A 1-meter long piece of string with an attached label that says  'hair diameter'
  9. Continue to model a mist water droplet that is 10 μm long. Cut a piece of string to the corresponding length and label it.
    Think about:
    How does the length of this string compare to the previous ones?

    A piece of string is measured with a ruler.
  10. The next piece of string represents an Escherichia coli bacterium that is 1 μm long. Such bacteria live in our gut and help digest the food we eat. Cut a piece of string to the corresponding length and label it.
    Think about:
    How long is an Escherichia coli cell in your scale model?

    A piece of string is measured with a ruler and marked at the 1-cm mark.
  11. Finally, model the coronavirus. Sars-Cov-2, which causes COVID-19, is about 120 nm long. Cut a piece of yarn that represents the length of the virus and label it.
    Think about:
    How big is the coronavirus in your model? Can you see it with your naked eyes?
    When looking at your model virus, remember that the coronavirus is only that big in your model where a pencil dot is 10 m long! In real life, it is 10,000 times shorter than this tiny piece of yarn!

    A cut piece of string that is 1-mm long.
  12. Compare the lengths of the five different strings. If you have the space, lay out all five pieces of string next to each other. You can zigzag the 10-m yarn if you run out of space.
    Think about:
    What do you notice when comparing the lengths of all five strings? Does your model give you a better perception of how big or small the coronavirus is?

    Five pieces of strings with different lengths are laid next to each other.
  13. Review the Scaling Chart again.
    Think about:
    Can you add other objects from the list to your scale model? How long would the piece of yarn be that represents the average length of a coffee bean?

Part 2 (Optional)

  1. Using a string to demonstrate the different sizes of objects allows you to model their sizes in one dimension only. In the next step, you will look at the sizes of objects in two dimensions (length and width). To reduce the complexity of the model, it is assumed that the objects have the same width and length—although this might not be true in reality. If you do not have enough space indoors, you can go outside into your driveway, yard, or to a park.
  2. Again, start with the biggest object in your model, the pencil dot. The string that represents the pencil dot should be 10 m long. Take the string to a place where you have lots of space, such as a park, your yard, or an empty parking lot. Use yarn or string to mark a square on the ground that is 10 m long and 10 m wide. In your scale model, this 10-m × 10-m square represents the size (length and width) of a pencil dot.
    Note: You can skip this step and move on to the size of the hair in Part 2 step 3 if you do not have the space or prefer to stay indoors.
    Think about:
    Can you think of an object that has the size of this square in real life?
  3. Take the next piece of string, which represents the average diameter of a human hair. If you made the 10-m × 10-m square, mark a new square inside the 10-m × 10-m square that is the size of a hair diameter in your scale model. If you did not lay out the 10-m × 10-m square, find a space where you can make a 1-m × 1-m square on the ground or wall and mark it with string, tape, or chalk (ask permission before marking or taping on the wall).
    Think about:
    Is the square big enough for you to stand in? How many of these squares do you think fit in the big square?
  4. Continue with the piece of string that represents the mist water droplet. Mark an area inside the 1-m × 1-m square that represents the water droplet.
    Think about:
    How much smaller is this square compared to the previous one?
  5. Next, mark a square that represents the Escherichia coli bacterium. Again, use the cut string from your model to make the square. If it is too difficult to mark the square with a string, cut a piece of paper that has the size of the E. coli bacterium in your model and place it inside the big square.
    Think about:
    How many of these bacterium cells do you think would fit into a mist water droplet?

    Several squares of different sizes marked with masking tape on the floor.
  6. Finally, make a pencil dot on a piece of paper and place it inside the big square. This pencil dot represents the coronavirus in your scale model. It is 1 square mm in size.
    Think about:
    How much smaller is the real coronavirus compared to the pencil dot in your scale model? How many coronaviruses could you fit inside a pencil dot?

    Different-sized squares marked on a floor with masking tape. Inside the smallest square a small red dot marks the coronavirus in the two-dimensional scale model.


Dispose of all the small pieces of string. You can reuse the longer pieces.

What Happened?

A virus, such as the coronavirus, is much too small to be visible with the naked eye; you cannot see them even with a light microscope! In order to make sense of the size of a virus, you built a scale model in this activity. In your one-dimensional scale model, a pencil dot, which is about 1 mm in length, became 10,000 times longer. This means that in your model, the string that represented the length of a pencil dot was 10 m long. When you modeled the other marked objects on the Scaling Chart (the hair diameter, mist water droplet, E. coli bacterium and the coronavirus), you probably noticed that the difference between their lengths was about one order of magnitude, which means that starting from the shortest object (the coronavirus), each following object was about 10 times longer than the previous one. As a result, in your model the pencil dot was 10,000 times longer than the coronavirus.

Based on that, the piece of yarn that represented the hair diameter in your scale model should have been 1 m long, the string for the mist water droplet 10 cm long, and the E. coli bacterium string 1 cm long. In your scale model, the coronavirus was 1 mm long, which is exactly the size of a pencil dot in real life. You probably noticed the differences in size even more in the two-dimensional scale model, which modeled the area of the different objects. In this model, the pencil dot should have been 10-m × 10-m, which equals 100 m2. In contrast, the coronavirus should have been 1 mm2. This means that in one pencil dot, you could fit about 10,000 × 10,000 (equals 100,000,000) coronaviruses!

Digging Deeper

Viruses are the cause for many infectious diseases, such as the flu or COVID-19. That is why it is important to protect yourself from exposure to these viruses in order to stay healthy. However, this can be difficult, as viruses are so tiny that they are not visible. In fact, most viruses are so small that you even cannot see them under a light microscope. Their size is in the nanometer range. The coronavirus Sars-Cov-2, which causes COVID-19, for example, has a size of about 120 nm (Figure 1). One nanometer (nm) is equal to one-billionth of a meter! That is shorter than the wavelength of visible light, which is in the range of 400–700 nanometers. Therefore, you need special instruments, such as electron microscopes, to make viruses visible. Because they are so tiny, health care providers have to wear special protective equipment, such as N95 masks, which are able to filter out such small particles.

 Image of a coronavirus particle with spikes. A scale bar across the virus particle reads 120 nanometers.
Figure 1. The size of a coronavirus particle is about 120 nanometers (nm).

Although we perceive viruses as very tiny, size is often relative, which means that in comparison to other objects a virus can be tiny or huge. For example, compared to atoms, which are 0.1–0.5 nm in diameter, a virus is pretty large. That is why we often compare the sizes of objects to get a better idea of how big or small they really are. A very useful tool for visualizing the size of objects is making a scale model. A scale model is a physical model of an object that is either larger or smaller than the actual size of the object. Such a model is specifically helpful for objects that are either too tiny to see, such as viruses, or too large to visualize, such as planets. An example of a scale model is a toy car or toy airplane. It looks exactly like a real car or airplane, but it is much smaller than a real one. For a scale model it is useful to know by how much the object is enlarged or shrunk compared to the real object. When you make a scale model of different objects at the same scale, you can compare their sizes, even though you might not able to see them in real life.

Measuring the size of an object can easily become complicated, depending on how many dimensions of the object you are considering (Figure 2). It is easy to measure the size of one-dimensional objects, such as a line or a piece of string, as these kinds of objects are usually measured in one dimension. The SI unit of measurement for length is meters (m). The SI measurement system is called the metric system, in which all measurements are related to each other by a multiple of ten. For example, 1 m is 100 times longer than 1 cm and 1000 times longer than 1 mm. Table 1 gives an overview of different measurement units for length and how they are related.

 Table showing the conversions factors from meters to centimeters, millimeters, micrometers, and nanometers.
Table 1. Measurement units (SI) for length.

Size measurements become more complicated for two-dimensional objects, where you not only measure an object's length, but also its width. In this case, you are calculating an object's area. The units used for area are typically m2, or cm2 (pronounced square centimeters). A square that has sides that are 10 cm long, has an area of 10 cm × 10 cm = 100 cm2. To measure the total amount of space that a three-dimensional object occupies you would need to calculate its volume, which, in addition to its length and width, also includes an object's height. The volume of a three-dimensional rectangular object (a cube) is determined by multiplying its length, width, and height. The units of measurement for volume is usually m3 (pronounced cubic meters). For example, a cube that has sides that are 10 cm long, would have a volume of 10 cm × 10 cm × 10 cm = 1000 cm3.

 Comparison between one-dimensional, two-dimensional, and three-dimensional size measurements. The left image shows a line that is 10 cm long, the middle image shows a square that is 10 cm square in size, and the right image shows a cube that is 10 cubic meters in size.
Figure 2. Measurement of one-, two-, and three-dimensional objects.
icon scientific method

Ask an Expert

Curious about the science? Post your question for our scientists.

For Further Exploration

  • Find other objects that would fit into your scale model. How big or small are they compared to the ones on the Scaling Chart?
  • Can you find some real-life objects that are the size of your modeled objects? For example, the pencil dot in your scale model is 10 m long. What real-life objects do you know that are 10 m long?
  • Can you find out how many viruses fit on a fingertip of about 1 cm2?
  • What other objects can you model using a scale model? To make a scale model of our solar system, review Science Buddies' How Big Are the Planets in Our Solar System?.
  • You can expand your virus scale model by one more dimension. Can you find materials to create three-dimensional models of all the objects on the Scaling Chart?


STEM Activity
11 reviews
Did you know that there are more planets than stars in our galaxy? All of these planets circle around a star, but only eight of them—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune—circle around the Sun—the star in our solar system. This activity explores the relative size of these eight planets. Is one bigger than the others, or are they all about the same size? Read more
STEM Activity
17 reviews
Have you ever wondered why we wash our hands with water and soap to get rid of bacteria and viruses? Good hand hygiene becomes especially important during viral outbreaks such as the COVID-19 pandemic! The CDC (Center for Disease Control) recommendations for handwashing say to scrub your hands for at least 20 seconds with water and soap. But how does soap kill viruses such as SARS-COV-2? In this activity, you will find out by experimenting with models of different virus types to see how they… Read more
STEM Activity
12 reviews
Did you know that the Voyager 2 spacecraft took 12 years to travel from Earth to Neptune, the furthest planet in the solar system? This sounds like a really long time! Maybe not that long if you consider what distance the spacecraft had to travel to get from Earth to Neptune. In this activity, you will make a model of the planets in the solar system and specifically model their distances to scale. Will it explain why the Voyager 2 took so long? Try it and see! Read more



Career Profile
Have you ever seen a car-crash safety test? The airbags always deploy just in time to save the test dummy, which is good news for those of us who travel in cars! What makes the airbag deploy properly? A tiny microelectromechanical systems (MEMS) device called an accelerometer. MEMS devices are used in many applications, from inkjet printer cartridges to neural probes. Typically 1 micrometer (µm) to 1 millimeter (mm) in dimension, MEMS devices require special techniques to make.… Read more
Career Profile
Imagine creating a new material, medicine, or electrical component that is too small to see. How would you design it? What could the new invention do? These are precisely the types of questions that nanosystems engineers answer every day. Nanosystems engineers design and build new technologies using the smallest building blocks, atoms, and molecules. Read more
Career Profile
Microorganisms (bacteria, viruses, algae, and fungi) are the most common life-forms on Earth. They help us digest nutrients; make foods like yogurt, bread, and olives; and create antibiotics. Some microbes also cause diseases. Microbiologists study the growth, structure, development, and general characteristics of microorganisms to promote health, industry, and a basic understanding of cellular functions. Read more
Career Profile
Doctors need information to decide if a person is healthy or sick, if a baby's earache is bacterial or viral, or if the man next door needs medication to lower his cholesterol and prevent a heart attack. The information often comes in the form of results from lab tests. Medical and clinical laboratory technicians are the people who perform these routine medical laboratory tests, giving the doctors the information needed to diagnose, treat, and prevent disease. Read more
Career Profile
Pharmacy technicians play a very important role at a pharmacy. Pharmacy technicians help pharmacists in many different ways. They make sure a pharmacy runs smoothly every day. They may have a role in customer service by taking personal information, answering questions, and taking payments. A technician can measure or mix drugs and package and label containers. They may also count all of the medications to let the pharmacists know if there are low stock items. Pharmacy technicians can work in a… Read more
Career Profile
Are you interested in working in the medical field to be an advocate and care for patients? If so, a nurse practitioner may be the career for you. Nurse practitioners require less school than a doctor, but with similar jobs. Nurse practitioners diagnose and treat illness as a part of a healthcare team or by themselves. Another important piece of their job is to teach patients and their families. They help patients stay healthy and teach them how to manage diseases. Nurse practitioners can work… Read more
Free science fair projects.