Perimeter and Surface Area Formulas

A circle is an ellipse where the distance from the center to the edge is constant.
Circumference ( c ) is the distance around the outside of the circle.
Diameter ( d ) is the distance of the line through the center of the circle from edge to edge.
Radius ( r ) is the distance from the center of the circle to the edge.
The ratio between the circumference and the diameter is equal to the number π.

d = 2r
c = πd = 2πr
Area = πr²