Try Pi

Let’s compare circumferences and diameters of balls and find Pi

You will need:

  • A few balls or other perfect sphere-shaped objects, like tennis, baseball, or basketball
  • A few feet of string and a ruler, or a measuring tape
  • Marker or pens – 2 different colors is ideal but not necessary
  • A calculator or person who is good at division

Here’s what to do:

If you are using the string method, mark the end of the string you are always going to start from when you measure, so that each time you measure a length, you start from the same end of the string.

Now, choose a ball. Measure its circumference, C, by using a string or measuring tape around the ball at its widest part around the middle. For the string method, mark with a marker just at the point before where the string would start to overlap. Now, measure the ball’s diameter, or d. Since the ball is a perfect sphere, however you look at it, the ball’s height and width are the same – measure either one to get the diameter! You can sandwich the ball between two hardcover books that you keep parallel, and measure the distance between the two books with string or measuring tape. Mark this second measurement on the string with either a different color marker or a slightly different width of mark, so you can keep track of which mark was which measurement.

Which is longer, the circumference, C or the diameter, d? Did that surprise you?

Try this:

  • Try folding the string or measuring tape at the the point of longest measurement back in thirds to the starting end, so that it is in three equal lengths. Where is your other mark? About 1/3 of the way, as if you could fit approximately three diameters lengths into the circumference length?
  • Try other size balls or spherical objects. Is the circumference always the longer measurement? Is it always about three times as long?
  • Now let’s try to find some Pi (also known as π) and see how this special number works. For one item you measured, divide the C (the circumference) by the d (the diameter) to get Pi. What number did you get? Is it close to 3.14? Many people celebrate the number Pi on Pi Day. March 14th! Now can you see why it is celebrated on this day?

Artists, architects, and engineers all use Pi, which is actually an infinite number (one that has no end) to help them calculate and create perfect circles or curves of specific sizes. This can help in everything from truck wheels to arches in buildings!

For more formulas that use pi, check out:
Pi is an infinite number! To see a million digits of pi, visit:

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