Human Body Ratios
Our bodies are amazing! They are full of mysteries and surprising facts like this one: did you know that you are about a centimeter taller in the morning, when you have just woken up after hours of lying down, than you are in the evening? You might never have noticed it. These interesting facts only reveal themselves when you look closely, measure, and compare. That is what this activity is about: measuring, comparing, and discovering how the human body measures up.
This activity is not appropriate 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.
Do you believe that human bodies come in all sizes and forms? When you start measuring, you will find our bodies show surprising similarities—and even more surprisingly, we can express these with mathematical concepts.
For one thing, our bodies are quite symmetrical. When you draw a vertical line down the center of a body, the left and right sides are almost mirror images of each other. Human bodies also show interesting ratios. Ratios compare two quantities, like the size of one part of the body to the size of another part, or to the size of the whole. An example of a human body ratio is a person’s arm span—the distance from the left hand’s middle fingertip to the right hand’s middle fingertip when stretching out both arms horizontally—to their height. This ratio is approximately a one-to-one ratio, meaning that a person’s arm span is about equal to their height. There are many more human body ratios; some are independent of age, and others change as we grow from a baby to an adult.
Wondering who would be interested in these ratios? Artists are avid users of human body ratios, as it helps them draw realistic-looking figures. They are also used in the medical world, as a sizable deviation from a human body ratio can indicate a body that does not develop according to expectations.
In this science activity, we will examine some human body ratios, and if you like, we can explore how they can help you draw more realistic-looking figures.
Extra: You have explored some ratios in your body and might wonder if these hold for other people as well. Do you think they hold for most people of your age? What about adults or babies, do you think these ratios hold for them, or would some be different? Make a hypothesis, find some volunteers, measure and compare. Was your hypothesis correct?
Extra: This activity uses pieces of yarn to compare lengths. You can also measure your height, arms span, the length of your femur bone, etc. with measuring tape, round the values, and write the ratios as fractions. Can you find a way to simplify these fractions?
Extra: Draw some stick figures on a paper. Can you apply some of the body ratios you explored (like the wing span to height or the head-to-body ratio) to the figures? Which ones look most realistic to you?
Extra: Ratios are all around us. Can you find other places where ratios play an important role? To get you started, think about the ratio of the quantity of one ingredient of a recipe to the quantity of another ingredient of that recipe. For avid bikers, can you find the ratios that correspond to the different gears on a bike?
Observations and Results
You probably found your arm span-to-height ratio approximately to be a 1 to 1 ratio, while the femur-to-height was approximately a 1 to 4 ratio. This is expected because on average and over a large age range, humans have an arm span that is roughly equal to their height and a femur bone that is roughly a quarter of their height.
The head-to-body ratio is a little more complex as it changes from a ratio of about 1 to 4 for a small child to about 1 to 8 for an adult. A five-year-old is likely to have a head-to-body ratio of about 1 to 6.
It is good to remember that these ratios are averages over a large group of people. Individual variations will occur. Some of these variations might even be used as an advantage, like being tall and having exceptionally long arms can be advantages when playing basketball.
More to Explore
Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Ratios, human body, mathematics
Explore Our Science Videos
Make Fake Snow - Craft Your Science Project
4 Easy Robot Science Projects for Kids
How to Make a Bristlebot