Calculate the circumference of a circle


As seen in the the number pi, the formula for the circumference of a circle is

C = 2 × π × r      or  C = π × D

If either r or D is known, you can calculate the circumference by simply substituting the known value for r in the formula. Use 3.14 for π.


Examples #1


Calculate C if r = 2 inches

C = 2 × π × r = 2 × 3.14 × 2 = 12.56 inches


Examples #2


Calculate C if r = 4 inches

C = 2 × π × r = 2 × 3.14 × 4 = 25.12 inches


Examples #3


If D = 10 cm, calculate the circumference.

You have two choices. You can first find r and then replace its value

r is half the diameter, so r = 10 divided by 2

r = 5 cm

C = 2 × π × r = 2 × 3.14 × 5 = 31.4 cm

Otherwise, you can just use the formula C = π × D

C = 3.14 × 10 = 31.4 cm


Examples #4


The circumference of circle A is four times the circumference of circle B

The diameter of circle B is 7. What is the diameter of circle A?

Let CA be the circumference of circle A

Let CB be the circumference of circle B

Let DA be the diameter of circle A

Let DB be the diameter of circle B

Since the ratio of circumference to diameter is the same for all circles, you can use the following proportion to solve this problem


CA / DA
   =   
CB / DB


Things that we know:

CA = 4 × CB

DB = 7

Replace these in the proportion

4 × CB / DA
   =   
CB / 7



If  
2 / 5
   =   
8 / 20
  then, 2 × 20 = 5 × 8



If  
4 × CB / DA
   =   
CB / 7
  then, 4 × CB × 7 = DA × CB



28 × CB = DA × CB

DA = 28 since 28 × CB = 28 × CB no matter what CB is


Examples #5

If the circumference of a circle is 50.24 inches, calculate r.

CA / DA
   =   
CB / DB


Things that we know:

CA = 4 × CB

DB = 7

Replace these in the proportion

4 × CB / DA
   =   
CB / 7



If  
2 / 5
   =   
8 / 20
  then, 2 × 20 = 5 × 8



If  
4 × CB / DA
   =   
CB / 7


Then, 4 × CB × 7 = DA × CB


28 × CB = DA × CB

DA = 28 since 28 × CB = 28 × CB no matter what CB is


Examples #5

If the circumference of a circle is 50.24 inches, calculate r.

calculate the radius






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