You sometimes see the word 'circumference' to mean the curved line that goes around the circle. Other times it means the length of that line, as in "the circumference is 2.11cm".

The word 'perimeter' is also sometimes used, although this usually refers to the distance around polygons, figures made up of straight line segments.

where: R  is the radius of the circle π  is Pi, approximately 3.142

where: D  is the diameter of the circle π  is Pi, approximately 3.142

where: A  is the area of the circle π  is Pi, approximately 3.142

Radius The radius is the distance from the center to any point on the edge. See radius of a circle

Diameter The distance across the circle. See Diameter of a Circle for more.

Other circle topics

General

• Circle definition
• Tangent
• Secant
• Chord
• Intersecting chords theorem
• Intersecting secants theorem
• Radius of a circle
• Diameter of a circle
• Area of a circle
• Concentric circles
• Annulus
• Area of an annulus
• Segment of a circle
• Area of a circle segment

Angles in a circle

• Inscribed angle
• Central angle
• Central angle theorem

Arcs

• Arc
• Arc length
• Adjacent arcs
• Major/minor arcs
• Intercepted Arc
• Sector of a circle
• Radius of an arc or segment, given height/width