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Circle Perimeter and Surface Area Formulas

Circle

Todd Helmenstine
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 = πr^{2}