Human Body Ratios

Here's how.

Key Concepts

Ratios, human body, mathematics

Introduction

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.

Background

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.

Materials

Yarn
Scissor
A hardcover book
A helper
Optional: Pen and paper
Optional: measuring tape

Procedure

To compare the length of different parts of your body to your height, we will first create a string the length of your height. Take off your shoes. The easiest way is to lie on the ground with your heels pressing against a wall. Look straight up and have your helper place a hardcover book flat on your head, resting on the ground. Get out from under the book and, together, span the yarn across the floor from the wall to the book. Cut the yarn just where it reaches the book. Now, you have a piece of yarn that is as long as you are tall. If lying on the ground is not possible, you can also stand flat on the floor against the wall and have the book rest against the wall.
First, we examine your arm span-to-height ratio. Your arm span is the distance between your fingertips when you stretch your arms out as far as they can reach. How do you think your height compares to your arm span? Would it be similar, way longer, or way shorter?
Now, stretch your arms out as far as they can reach. Your arms will be parallel to the ground. Hold one end of the piece of yarn you just cut off with the fingertips of your left hand. Let your helper span the yarn towards the tip of your right hand’s middle finger. Is piece long enough, way longer, or way too short? What does this tell you about how your arm span compares to your height?
For most people, their arm span is about equal to their height. Mathematicians say the arm span-to-height ratio is 1 to 1: your arm span goes 1 times into your height.
Now, let’s explore another ratio: the length of your femur bone to your height. The femur bone is the only bone in your thigh. To measure its length, sit down and span a new piece of yarn over your thigh from the hip joint to the edge of your knee, and cut the yarn there.
Make an estimate. How many times would this piece of yarn go into the piece that is as long as you are tall? Can you find a way to test your estimate?
There are several ways to compare the length of the two pieces of yarn. You might cut several pieces of the length of your shorter string, lay them end-to-end next to your longer piece, and count how many you need. Another way is to fold the longer string into equal parts so that the length of the folded string equals the length of the shorter string. The number of folds needed is exactly the number of times your shorter string goes into your longer string.
Did you see that the length of your femur bone goes about four times into your height? You can also say that if you divide your height in four equal pieces, you have the length of your femur bone, or the length of your femur bone is one fourth of your height. Mathematicians call this a 1 to 4 ratio.
Now, let’s move on to a ratio that might help you make more realistic drawings: the head-to-body ratio. How many times would the length of your head fit into your height? Maybe four, six, or eight times? To test six times, fold the yarn with length equaling your height into six equal pieces. Have your helper place a book flat on your head and hang the folded string from the side of the book. If the other end of the string is about level with your chin, your height would be about six times the length of your head, or your head-to-body ratio would be 1 to 6. Which number of folds fits best for you?
There are many more body ratios you can explore, like the circumference of your head compared to your height, the length of your forearm to the length of your foot, or the length of your thumb to the length of your hand. Use pieces of yarn to measure, compare, and detect these and/or other ratios of your body.

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

Simple Ratios of the Human Body, from Bedtime Math
Stepping Science: Estimating Someone's Height from Their Walk, from Scientific American
Earthquake-Proof Engineering for Skyscrapers, from Scientific American
Size of a Human: Body Proportions, from the Physics Factbook

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Credits

Sabine De Brabandere, PhD, Science Buddies

Reviews

Key Concepts

Ratios, human body, mathematics

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