If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Course: 7th grade (Illustrative Mathematics)>Unit 3

Lesson 1: Lesson 2: Exploring circles

Labeling parts of a circle

Radius, diameter, center, and circumference--all are parts of a circle. Let's go through each and understand how they are defined. Created by Sal Khan and Monterey Institute for Technology and Education.

Want to join the conversation?

• What is the center?
(34 votes)
• Imagine radii One end point is on the circumference. The other point is shared by all the radii and is equidistant from any point on the circumference and. IS called the centre of the circle
(21 votes)
• why is half of the diameter called the radius
(22 votes)
• it not the line through the circle is the diameter and the radius is half of the circle
(4 votes)
• I hate circle!
(16 votes)
• same
(6 votes)
• why do we need to learn this?
(15 votes)
• i want to be an software engineer, not a circle engineer.
(15 votes)
• Math and science is everything
(1 vote)
• at what dont undertsand bro
(5 votes)
• The circumference of a circle is basically the distance around a circle. For example, if you had a park or other outdoor area that was shaped in a perfect circle, and you walked all the way around the edge of it, you would have walked along the circumference of the circle. Basically, you can think of the circumference as the perimeter of a circle.
(12 votes)
• I am confused why do we use pi in this equation
(3 votes)
• I think that's because almost all equations (if not all) relating to circles use pi
(10 votes)
• what does the 'd' stand for?
(4 votes)
• D stands for diameter
(3 votes)
• this helped out a lot thank you
(5 votes)
• Is RADII singular form of RADIUS
(3 votes)
• Radii (pronounced ray-dee-eye) is the PLURAL form of radius.
(5 votes)

Video transcript

Draw a circle and label the radius, diameter, center, and the circumference. Let me draw a circle. And it won't be that well drawn of a circle, but I think you get the idea. So that is my circle. I'm going to label the center over here. I'll do the center. I'll call it c. So that is my center. And I'll draw an arrow there. That is the center of the circle. And actually, the circle itself is the set of all points that are a fixed distance away from that center. And that fixed distance away that they're all from that center, that is the radius. So let me draw the radius. So this distance right over here is the radius. That is the radius. And that's going to be the same as this distance, which is the same as that distance. I can draw multiple radii. All of these are radii, the distance between the center and any point on the circle. Now, a diameter just goes straight across the circle, going through the center. From one side of the circle to the other side, I'm going through the center. It's essentially two radii put together. So for example, this would be a diameter. You have one radii, than another radii, all one line, going from one side of the circle to the other, going through the center. So that is a diameter. And I could have drawn it other ways. I could've drawn it like this. That would be another diameter. But they're going to have the exact same length. And finally, we have to think about the circumference. And the circumference is really just how far you have to go to go around the circle. Or if you put a string on this circle, how long will that string have to be? So what I'm tracing out in blue right now, the length of what I'm tracing out, is the circumference. So right over here, that is the circumference. And we're done.