Circumference of a Circle
Math 10-3
Ch.3 Measurement
Properties of Circles…

Yesterday we talked about the perimeter
of various 3 sided and 4 sided shapes.
How do we find the perimeter of a circle?

First, let’s review the properties of the
circle:

*All the points on a circle are equidistant
(the same distance) from the CENTER
of the circle.
Properties of Circles…

*A line that passes through the center of
a circle and touches the edge of the circle
on both sides is called the diameter.
Properties of Circles…

*A line that starts at the center of the
circle and touches an outside edge is
called the radius.
Diameter and Radius…

*The radius can be calculated by dividing
the diameter by 2
r  d 2

*The diameter can be calculated by
multiplying the radius by 2
d  2r

*The circumference of a circle is the
perimeter of the circle. It can be
calculated with the formula : C   d

Where
◦ C = circumference (perimeter)
◦
= “pi” a constant that is 3.14159….
◦ d = diameter

Ex1. What is the circumference of a circle with a
diameter of 8 cm?
C=?
 d= 8cm

C d

C = x 8

C = 3.14 x 8

= 25.12 cm
*note: we will use the estimation of 3.14 for pi in this course
Ex2. What is the circumference of a circle with a radius
of 2.5 mm?
C=?
 d= ?
 r = 2.5 mm
 *first we must find the diameter of the
circle.
d=2xr
 d= 2 x 2.5 mm = 5 mm

Ex2. What is the circumference of a circle with a radius
of 2.5 mm?
C d

C= x5
 = 3.14 x 5


= 15.7 mm
Ex3. The circumference of a circle is 52cm. What is the
diameter?


C = 52 cm
d=?

*We must perform opposite operations (algebra!) to
calculate the diameter!


First, fill in the formula with what we know:
52 cm = 3.14 x d

What is opposite of multiplying by 3.14? Dividing by 3.14 on
the other side!


52 cm 3.14 = d
d = ~16.56 cm

C d