Circumference and Pi

Contributor: Erika Wargo. Lesson ID: 12576

What do pi and pie have in common, besides two letters? Pi helps you calculate the circumference of a circle, and helps bakers make pies! Learn the parts of a circle and how to use them to measure!


Geometry, Plane Geometry (2D)

learning style
Kinesthetic, Visual
personality style
Grade Level
Middle School (6-8)
Lesson Type
Quick Query

Lesson Plan - Get It!

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Let's say you are a swimmer practicing for the Olympics. You need to swim a certain distance, so you lap around the edge of your round swimming pool.

  • How do you calculate the distance around the swimming pool?
  • What information would you need to know?

Many objects that you use and see every day are circles.

A circle is made up of points that are the same distance from the center. There are three main parts to a circle.

  • Do you know the names of the three parts labeled on the circle below:

Check and see how you did.

Click on each + sign to read a description of each part of the circle:

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Now that you are familiar with the parts of a circle, learn more about calculating the circumference of a circle with an activity.

Complete the following steps:

  1. Go to Circumference (Scholastic StudyJams!) and click on the Step-by-Step button to watch how to calculate the circumference of a circle.
  1. Click on the NEXT and SHOW ME buttons to progress through the steps.
  1. Click on WATCH OUT! to review facts about pi.

Next, watch this video from Don't Memorise to understand where pi came from and why we use it.

What is the Circumference of a Circle? | Perimeter of a Circle | Don't Memorise:

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After you have completed the activities above, consider the following:

  • What is circumference?
  • What is the formula for finding circumference?
  • What value of pi should be used in the formula?
  • If you know the radius of a circle, how do you calculate the diameter?

At the beginning of the lesson, you were presented with a question about a circular pool:

  • How do you calculate the distance around the swimming pool?
  • What information would you need to know?

circle swimming pool

  • What information do you think is needed in order to solve the distance around the pool?

Review the example below:

  • If the radius of a circular pool is 5 feet, what is the circumference of the pool?

To find the circumference of a circle, use the formula, C = pi (Π) x D.

You are given a radius of 5 feet, so to find the diameter, multiply the radius by 2. 5 x 2 = 10, so the diameter is 10.

Since the diameter is 10 and pi is 3.14, 10 x 3.14 = 31.4 feet. The circumference, or distance around the circular pool, is 31.4 feet.

One way to remember the formula for circumference is to think about a cherry pie:

"Cherry pie is delicious" or C = Π x D.

In the formula, think of the "C" for circumference as "cherry," the Π symbol as "pie," and the "D" for diameter as "delicious."

cherry pie

In your math journal, write a response to the following:

  • Draw and label the parts of a circle: diameter, radius, circumference.
  • What information is needed to calculate the circumference of a circle?
  • Explain a time you would need to find the circumference of a circle, other than the example given in this lesson about the pool.

In the Got It? section, you will practice calculating the circumference of a circle as you complete interactive practice and move around a life-size circle.

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