Celebrate Pi Day by Learning the Secret of the Circle

Reading Time: 2 minutes

circle-300x257Hands-on projects are great learning tools for kids, especially when they involve the word ‘secret’. I can’t think of a better way to celebrate Pi Day (March 14 or “3.14”) with kids than to help them unlock the circle’s secret and discover Pi!  This fun activity uses common office supplies and household materials and is easy even for younger kids.

Materials needed

  • A compass or two different sized round containers from which to trace around the bottom
  • A sharpened pencil
  • A ruler
  • A calculator
  • Long lengths of heavy string or yarn
  • Scissors to cut the string


Follow these steps to discover the secret of the circle.

Tip: Use a compass to draw two different sized circles. It’s best to draw them of drastically different sizes.

  1. Lay out the length of string around the outside edge of the first circle, cutting it to fit precisely once around the outer edge.
  2. Now measure the length of string using the ruler and record this measurement as the circumference of the large circle.
  3. Draw a line through the center of the large circle and extend it to the edges of the circle. This is the diameter.
  4. Measure the diameter of the large circle using your ruler and record this number as the diameter of the large circle.
  5. Now, calculate the ratio of C/d (circumference divided by the diameter). A calculator is best for younger students so that they understand the concept of Pi even if they haven’t yet learned division.
  6. Repeat these steps for the smaller circle.

The secret of the circle is that no matter what size circle you start with, dividing the circle’s circumference by its diameter (C/d) always yields Pi!  Note that this activity will likely not  yield precisely Pi because of the somewhat crude measuring techniques.  However, repeating this exercise for many different sized circles reveals the pattern that holds the circle’s deepest secret.

Further reading

Here is a bit more information on Pi.

Wikipedia’s Pi information.

I found this lesson in one of my favorite math teaching aids, ‘Math Wizardry for Kids‘.


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