Objective:

To calculate the circumference of a circle.

General:

• The formula for calculating the circumference of a circle is:

• Knowing or calculating the diameter, we can use the D and DF scales or the C and CF scales to read the circumference of the circle.

Method:

• Calculate the diameter of the circle (this may be given or you will be given the radius).
• Move the cursor to the diameter on the D scale.
• Read the area from the DF scale.

or

• Calculate the diameter of the circle.
• Move the cursor to the diameter in the C scale.
• Read the area from the CF scale.

 

Example 1: Calculate the circumference of a circle with diameter 3 mm.

• Move the cursor to D3
• Read the result DF9.42

Hence the circumference of the circle is 9.42 mm.

 

Example 2: Calculate the circumference of a circle with radius 4".

• As the radius is 4", we double it to obtain the diameter of 8".
• Move the cursor to C8
• Read the result CF25.1

Hence the circumference of the circle is 25.1".

 

Practice Questions:  Calculate the circumference of the following circles.

1. d=5.2 cm (Ans: 16.3 cm)
2. d=22.8 m (Ans: 71.6 m)
3. r=12.6" (Ans: 79.2")
4. r=5.6 mm (Ans: 35.2 mm)