1.
Circumference

2. Vocabulary Center – the point in the middle of the circle
Circumference – the distance around a circle.
Diameter – distance across a circle through its center.
Radius – the distance from the center to any point on the circle.

3. Hula Hoops The table shows the approximate measurements
Size
Radius
Diameter
Circumference
Child
14 in.
28 in.
88 in.
Adult
20 in.
40 in.
126 in.
Hula Hoops
The table shows the approximate measurements
of two sizes of hula hoops.
Describe the relationship between the diameter and radius of each hula hoop.
Describe the relationship between the circumference and diameter of each hula hoop.
The diameter is twice the radius.
The circumference is about three times the diameter.

4. Radius and Diameter The diameter of a circle, d, is twice its radius.
The radius, r, of a circle is half of its diameter.
d = 2r
r =

5. Radius & Diameter Examples
The diameter of a circle is 14 inches. Find the radius.
The radius of a circle is 8 feet. Find the diameter.
14 in.
The radius of the circle is 7 inches.
8 ft.
The diameter of the circle is 16 feet.

6. Practice Problems
Find the radius or diameter of each circle with the given dimension.
1. d = 23 cm r = 3 in.
3. d = 16 yd r = 5.2 m
The radius is 11.5 centimeters.
The diameter is 6 inches.
The diameter is 10.4 meters.
The radius is 8 yards.

7. Circumference
The circumference of a circle is equal to π times its diameter or π times twice its radius.
C = π(d) or C = 2(π)(r)
Example: Find the circumference of a circle with a diameter of 21 inches. Round your answer to the nearest whole number.
C = (π)(d)
C = (π)(21)
C = …
C = 66 inches
21 in.
The circumference of the circle is 66 inches.

8. Practice Problems
Find the circumference of each circle. Round your answers to the nearest tenth.
5 ft.
70 in.
C = 2π(r)
C = 2π(5)
C = 10 π
C = …
C = 31.4 feet
C = π(d)
C = π(70)
C = …
C = inches

9. Word Problem
The Belknap shield volcano is located in the Cascade Range in Oregon. The volcano is circular and has a diameter of 5 miles. What is the circumference of this volcano? Round your answer to the nearest tenth.
C = π(d)
C = π(5)
C = …
C = miles