The diameter of a circle is the length of the line through the center and touching two points on its edge. In the figure above, drag the orange dots around and see that the diameter never changes.

Sometimes the word 'diameter' is used to refer to the line itself. In that sense you may see "draw a diameter of the circle". In the more recent sense, it is the length of the line, and so is referred to as "the diameter of the circle is 3.4 centimeters"

The diameter is also a chord. A chord is a line that joins any two points on a circle. A diameter is a chord that runs through the center point of the circle. It the longest possible chord of any circle.

where: R  is the radius of the circle

where: C  is the circumference 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. As you can see from the figure above, the diameter is two radius lines back to back, so the diameter is always two times the radius. See radius of a circle

Circumference The circumference is the distance around the edge of the circle. See Circumference 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