Introduction: Circumference of a Circle

Picture of Circumference of a Circle

In this Instructable, we will dive into the exciting world of Circles.
Specifically the Circumference of a Circle.

Pencil (needed for it's erase-ability)
Calculator (to check your answer)

Step 1: Circle

Picture of Circle

Some basics of the Circle:

Circumference - the distance around the circle (aka the perimeter of the circle)
Radius - the distance from the center to any point on the circle
Diameter - the distance from a point on the circle to another point on the circle going
through the center (also twice the length of the Radius)
Pi - the symbol that represents the value 3.14 (actually Pi is a never ending number, but for our purposes 3.14 will be fine)

Step 2: Circumference

Picture of Circumference

Now to find the Circumference

Formulas for Circumference:
C = 2*Pi*r
C = Pi*d

r - radius
d - diameter

General Steps for solving:
1) write the formula you will use
2) write the information Given
3) substitute the given information into the formula
4) follow your Order of Operations (PEMDAS) to simplify and reduce
5) circle your answer
6) check your answer

Example 1:
Given: circle with a radius = 4 cm
1) C = 2*Pi*r (choose this formula because you are given a radius)
2) r = 4
3) C = 2*Pi*4
4) C = 8*Pi
4) C = 8*3.14
5) C = 25.12 cm (don't forget your units)

Example 2:
Given: circle with a diameter = 12 in.
1) C = Pi*d (choose this formula because you are given a diameter)
2) d = 12
3) C = Pi*12
4) C = 3.14*12
5) C = 37.68 in. (don't forget your units)


TheDoctor11 (author)2013-07-07


TheDoctor11 (author)2013-07-07

Ive got 88! Memorized it fir a contest.


Plz tell me if i messed up

notanemo (author)2009-10-04

off the top of my head, pi is equal to 3.141592653589793. i have it to a million on my comp

mrpesas (author)notanemo2010-03-13

I only have it memorized to 3.14159.  I like to show my students the websites that let you search for your b-day, phone number, ss#, etc in pi
like this

joen (author)2010-03-13

in example 2 of step 2 you show that the radius is 12 in. Don't you mean the diameter?

mrpesas (author)joen2010-03-13

You are correct.  Thank you for pointing it out.  I fixed the page. :-)

thepelton (author)2010-03-13

I know it's not as precise, but 22/7 works for a close guesstimate.

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