Hi, I'm Steve Jones and I'm going to tell you how to calculate the circumference of a circle. Now, here you can see our circle, and it's center o, and radius r. Now, the circumference of a circle is the distance around the outside. That's what we mean by the circumference. There is a simple formula for that. The circumference is equal to two times pi, times r. R is the radius. So, some people use a different formula, pi times d, where d is the diameter, that's two r. It's exactly the same. So, all we have to do, is put in the numbers, into the formula. So, if we put a radius of three centimeters, and pi, of course, as we know, is approximately three, so we'll just put a value of three for it. Then we could do our calculation. We're looking for the circumference, is the radius, three times pi, which is also approximately three, multiplied by two, and that gives us, three times three is nine, times two is eighteen, and of course, it is centimeters. This is a distance around the outside of the circle. So, it is in centimeters. So, that is the simple way to calculate the circumference of a circle.