Can you see pi?

English: Animation of the act of unrolling cir...

Animation of the act of unrolling circumference of a circle having diameter 2, illustrating the ratio π. (Photo credit: Wikipedia)

Yesterday, March 14, was Pi Day.

Today, we are going to see how long pi is.


  • Round salt container (can use any cylinder. A toilet paper roll works, too)
  • String
  • Measuring tape
  • Scissors
  • Calculator or pen and paper (optional)
  • Second cylinder, a different size from first (optional)


  • Wrap the string around the salt container.
  • Cut the string where it meets the end of the string, so you have a piece that is as long as the salt container is around
  • Lay the string you have cut across the top of the salt container
  • Cut off the string the length the container is across
  • Keep cutting pieces of string the length of the container across, until the remaining piece of string is too short to go across the container.
  • Optional: do the same thing with a different size cylinder, like a cardboard paper towel roll.

What Should Happen?

You should have three pieces of string the same length and one very short one.

Pi is the ratio of the distance around a circle, or its circumference and the distance across a circle, or its diameter.

It is always 3.14, rounded off, just like your three pieces of string, plus a little.

Line up your four strings, evenly, and you will see pi, 3 + a little.

The number of places after the decimal point can be extended endlessly, without the number pattern repeating.

It does not matter how big or small the cylinder or circle is.

You can try it with several different sizes of cylinders.

The diameter will always be in the same ratio to the circumference, 3.14, or pi.

This is called a constant.

What Is It Used For?

Because pi is a constant, if you know either the diameter or the circumference of a circle, you can calculate the value of the other using pi.

Another way to use pi is to figure out what your hat size is.

An adult’s head is generally 21 to 25 inches around.

If you measure around an adult’s head and divide by pi using paper and pencil or a calculator, you will get their hat size.

Thanks to for this activity


Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers


Related posts

Enhanced by Zemanta