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A circle is a geometric shape where the distance from the outer border to the centre is the same everywhere. This distance is the radius of the circle. The word circle is derived from the Latin word circulus.
The distance between two points on the perimeter is greatest when the straight line between them goes through the centre. This line is called the diameter and is twice the length of the radius. The perimeter of a circle is often called the circumference.
The perhaps most famous mathematical constant π (pi) has a very close relationship with circles. Pi is defined as the ratio between the circumference and the circle's diameter. This ratio is the same for circles of all sizes.
Pi can among other things be used to calculate the circumference and area of a circle.
Since the diameter, circumference and area only depends on the radius it is enough to know one of these properties to be able to calculate the rest. You can do the calculations yourself using a bit of calculus, or you can let the tool on this page do it for you.