Circumference
Warm Up
Find the circumference of each circle. Use 3.14
or for π. Round to the nearest hundredth, if
necessary.
For which problems did you use
Explain your choice.
for π?
Complete the table.
Shape
Perimeter
Formula
Side Length
Square
s=7
Rectangle
l = 5,
w=2
Equilateral
Triangle
s = 2.5
Circle
Circumference
formula
d=7
r = 3.5
Perimeter
Circumference
• The diameter of a circle is twice the
radius.
• The ratio of the circumference of a circle
to the diameter is = .
• Common approximations for π are 3.14
and .
• There are two equivalent formulas for
the circumference of a circle: C = πd and
C = 2πr, where d is the diameter of the
circle and r is the radius of the circle.
Misty has a circular duck pond. If fencing
costs $3.50 a foot, how much would it cost
to put a fence around the circumference of
the pond? Find the answer using both 3.14
for π and for π. Then explain which was
easier to use in this situation.
A circular wading pool at a park
has a radius of 21 feet. Find the
circumference of the pool.
Use for π.
A circular concrete patio has a
circumference of about 1,400 feet. A
gardener walks directly across the patio
along a radius at a rate of 1.5 feet per
second. How long does it take him to get
from the edge of the patio to the plant at
the center?
The Ferris wheel shown makes 12
revolutions per ride. How far would
someone travel during one ride?
A map of a public park shows a circular
pond. There is a bridge along a
diameter of the pond that is 0.25 mi
long. You walk across the bridge,
while your friend walks halfway
around the pond to meet you at the
other side of the bridge. How much
farther does your friend walk?
Sam is placing rope lights around
the edge of a circular patio with a
diameter of 18 feet. The lights
come in lengths of 54 inches. How
many strands of lights does he
need to surround the patio edge?
A circular path 2 feet wide has an
inner diameter of 150 feet. How
much farther is it around the outer
edge of the path than around the
inner edge?
Exit Ticket
For 1–3, find the circumference of each circle. Use 3.14 or
for π. Round to the nearest hundredth if necessary.
1. a circle with radius 63 mm
2. a circle with diameter 6.2 in.
3. a circle with diameter 18 in.
4. A circular satellite dish antenna has a circumference of about
62.8 inches. Find its diameter.
5. The diameter of a bicycle’s tires is 2.5 feet. How many
revolutions of the tires does it take to travel 1 mile? Use
5,280 feet = 1 mile. Round to the nearest tenth.
6. The Jones family wants to enclose their circular garden,
which has a radius of 62.5 ft., with a fence. To the nearest
foot, what is the minimum length of fencing they need to
buy? If the fence costs $2.50 per foot, what will the total
cost be?