Simple Ratios of the Human Body

Here's your nightly math! Just 5 quick minutes of number fun. Read a cool fun fact, followed by math riddles at different levels so everyone can jump in. Your kids will love you for it.

Simple Ratios of the Human Body

September 26, 2013

No doubt you’ve counted your child’s fingers and toes more times than you can remember. Here’s a fun spin on basic “body math” that teaches the concept of ratios in a fun way using the human body as your guide. All you need is a tape measure, some string or rope, a piece of paper, a pen or pencil, and, of course a few bodies.

Femur Bone Connected to the…

A ratio compares the relation in degree, size or number of two or more things. Explore ratios through femur (thigh bone) and height measurements. In an adult, the femur is said to be approximately a quarter of a person’s height.

To test this concept out, have your child measure the length of his femur and then measure his total height, writing the measurements on a piece of paper. Then, ask him what kind of math he can use to figure out whether his femur is a quarter the length of his height or not. He can multiply the femur length by four and see if it equals his height. But to find the ration, divide his height by the length of one femur and see the resulting number equals or is close to four.

Now offer yourself up as an experimental subject. Let him perform the same measurements and math on you! Invite other friends and family to take this little test and see who has a perfect 1:4 ratio of femur length to total body height.

Big Headed Math

While you will find variations in head circumference-to-height ratios, the majority of people will have about a 1:3 head-to-height ratio.

Ready to test this one out?

Ask your child to measure out a piece of string or rope that matches her height, and then have her write down the length of the string. Ask her to take a look at the string and guess how many times she can wrap it around her head. She should write her prediction down as well. Finally, ask your child to wrap the string around her head until she has no more string left. How many times did she manage to wrap it around? For most people, the answer will be 3 times, proving the 1:3 ratio, but have your child write whichever ratio she came up with. If she managed 4 times around, the ratio of her head circumference to her height is 1:4. Try this with friends and family. Your child will amaze them with his knowledge of math and the secrets of our bodies that are hidden in plain sight.

A Handy Ratio

Ask your child to eyeball the distance between his thumb and his pinky finger when his hand is outstretched. It probably doesn’t look very long, but believe it or not, it’s about the same as the distance between his elbow and his wrist, a 1:1 relationship. Ask your child to measure the difference between his thumb and his pinky and then his wrist and his elbow to see this relationship. Then explain that the ratio of this relationship is 1:1 since they are the same distance.

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Kim Moldofsky

Kim Moldofsky is a mom of teen boys in the Chicago area. She blogs at and hosts a the popular monthly #STEMchat on Twitter where parents and educators share ideas and resources to raise STEM-loving kids.

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