How to Figure Out the Perimeter of a Circle

Video Transcript

"Let's take a look at the perimeter of a circle. So the funny thing is, perimeter is what we usually use, that is the word we usually use when we are talking about the length around a shape, like what is the perimeter of this rectangle. You would add up this side plus that side plus this one plus this one. You would add up the lengths as though this was a fenced-in area, add up the length of the fence, and that would be the perimeter. So in a circle, when we want to talk about that perimeter, when we want to talk about how long this fence is around this interior, we want to know how long this line is, this curving circle is, we do not actually use the word perimeter ordinarily. We call it circumference. But it is the same thing. So to figure out the circumference of a circle, we know need to know the radius or the diameter. And either one works because actually, you might know the radius, let's call it three, is always half the diameter. Now the way to the diameter, the length from one side to the other through the center, is always double the radius, so if that was six, this would be three. And the circumference is just 2 pi R or pi D. So 2 pi R means two times pi, which is 3.14 rounded, times the radius, in this case three. So we would say it is 6 pi. Or you could use diameter, which since the diameter is six would also be 6 pi. And the reason there is two formulas is just because 2 R, as we were just saying, is D, and that is why they both work."