Calculating Circumference

Calculating Circumference

Important Vocabulary

• Radius: a line segment from the center of a circle to the outside

• Diameter: a line segment going across an entire circle, passing through the middle

• Circumference: the distance around the outside of the circle, its perimeter

• Pi: the ratio of the circumference of a circle to its diameter.

Multiple Choice

What is the radius of the circle?

• 50 cm

• 100 cm

• 150 cm

• 500 cm

Multiple Choice

What is the diameter of the circle?

• 50 cm

• 100 cm

• 150 cm

• 500 cm

Multiple Choice

What is the radius of the circle?

• 12 in

• 24 in

• 144 in

• 6 in

Multiple Choice

What is the diameter of the circle?

• 12 in

• 24 in

• 144 in

• 6 in

Calculating Circumference

• There are two formulas that you could use to calculate the circumference. Notice that the formulas mean the same thing!

•  $C=\pi d$  

•  $C=2\pi r$  

Calculating Circumference

Look at the two examples.

NOTICE that different formulas were used because we were given the radius in #1 and the diameter in #2.

Use a calculator to make sure the answers are correct.

Multiple Select

Which of the following formulas are used to calculate the circumference of a circle? (Select ALL that apply.)

• $C=2\pi r$

• $C=8\pi r$

• $C=\pi d$

• $C=\pi r$

Multiple Choice

What is the circumference of this circle? Round your answer to the nearest hundredth and use 3.14 for pi.

 $C=2\pi r$  
 $C=\pi d$  

• 37.68 in

• 12 in

• 6 in

• 75.36 in

Multiple Choice

What is the circumference of this circle? Round your answer to the nearest hundredth and use 3.14 for pi.

 $C=2\pi r$  
 $C=\pi d$  

• 18 in

• 56.52 in

• 9 in

• 113.04 in

Multiple Choice

What is the circumference of this circle? Round your answer to the nearest hundredth and use 3.14 for pi.

 $C=2\pi r$  
 $C=\pi d$  

• 6 in

• 3 in

• 18.84 in

• 37.68 in

Multiple Choice

What is the circumference of this circle? Round your answer to the nearest hundredth and use 3.14 for pi.

 $C=2\pi r$  
 $C=\pi d$  

• 18 in

• 56.52 in

• 9 in

• 113.04 in

Multiple Choice

You collect coins. One of your favorite coins is a silver-colored coin showing a man’s portrait. The radius of the coin is 12 millimeters. What is the circumference of the coin? 

Round your answer to the nearest hundredth and use 3.14 for pi.

• 12 mm

• 24 mm

• 75.36 mm

• 37.68 mm

Poll

Rate your understanding of the following target:

I can find the circumference of a circle.

• 0 - I don't get it AT ALL

• 1 - I still need some help

• 2 - I can do it, but I might need a reminder

• 3 - I can do this on my own

• 4 - EASY. I can teach someone else how to do this.