The document explains how to calculate the area of a circle. It first discusses the difference between circumference and area. It then shows how a circle can be cut up and rearranged to form a parallelogram. This allows the area formula for a parallelogram, Area = Base x Height, to be used. The base is equal to half the circumference which is equal to radius x pi. The height is equal to the radius. Therefore, the area of a circle is equal to the radius squared times pi. Some examples are provided to illustrate calculating the area of different circles using this formula.

1. How to calculate the
area of a circle.
It’s as easy as pi.

2. Let’s first make sure that we
understand the difference
between circumference and
area.

3. The circumference of a circle
is the perimeter of the circle.

4. Imagine that the circle is
straightening itself out.

8. The length of this line
segment is the
circumference
of the
circle.
314 cm

9. The circumference is the same
length as 3 diameters plus .14
of another diameter.

10. So, circumference = diameter x 3.14

15. Does this look familiar?

16. O.K., now it’s time to
move forward with some
new stuff.

17. How in the world would you
find the area of a circle?

18. Remember, area is always
measured in square units.

19. Remember that the area of a
rectangle is length x width because
you’re calculating the total number
of squares inside of the rectangle.
2
4

20. That’s fine and dandy, but a circle is
not a polygon. It does not have
straight sides; it has curves.

21. How are we going to get
around these curves?

22. Imagine chopping up the circle as if
it were a pizza.

23. Now, let’s rearrange our
“pizza” into another shape.

31. PRESTO!

32. Great Mr. Dunlap! But what
in the world is this?

33. Believe it or not, this is really
our “friend” the parallelogram.
And, we know how to
calculate the area of a
parallelogram.

34. Rats! He always has an
answer for everything.

35. Area = Base x Height
Base
Height

36. To find the area of the circle (which
is now a parallelogram), we just
need to multiply the Base by the
Height.
Base
Height

37. Base
Radius
Wait a minute! The height of this
“parallelogram” is really the radius
of the circle.

38. 1/2 of Circumference
Radius
Wait a minute! The Base is really
1/2 of the circumference.

39. 1/2 of Diameter x π
Radius
Wait a minute! The circumference is
really Diameter x π

40. Radius x π
Radius
Wait a minute! 1/2 of a Diameter is
really a Radius.

41. Base
Height
So if we multiply the Base x Height

42. We are really multiplying
Radius x Radius x π
Radius x π
Radius

43. Practice Time!

44. 1) Now let’s try this formula.
Find the area of this circle.
5 cm

45. 5 x 5 x 3.14 = 78.5 square cm
5 cm

46. 2) Find the area of this circle.
6 cm

47. 6 x 6 x 3.14 = 113.04 square cm
6 cm

48. 3) Find the area of this circle.
9 cm

49. 9 x 9 x 3.14 = 254.34 square cm
9 cm

50. 4) Find the area of this circle.
20 cm

51. 10 x 10 x 3.14 = 314 cm2
20 cm
Make sure
that you use
the radius of
the circle.

52. 5) Find the area of this circle.
14 cm

53. 7 x 7 x 3.14 = 153.86 cm2
14 cm
Make sure
that you use
the radius of
the circle.

54. 6) Find the area of this circle.
22 cm

55. 11 x 11 x 3.14 = 379.94 cm2
22 cm

56. Area = Radius x Radius x π
It’s as easy as pi.