That's the distance right across the centre of the circle. The diameter is always twice as long as the radius. And now the circumference. That's the distance all the way around the side of the circle.

It’s important to note that the radius is not a perfect circle but a series of straight ... You can move the point around the map to adjust the radius or delete it entirely by clicking the ...

A model of the famous toy known as a spirograph. A circle of radius r (see blue circle below) is allowed to roll around the inside of a circle of radius R (see black circle below). We assume that the ...