The Geometry of Circles
Definitions and formulas for the radius of a circle, the diameter of a circle, the circumference (perimeter) of a circle, the area of a circle, the chord of a circle, arc and the arc length of a circle, sector and the area of the sector of a circle
Just scroll down or click on what you want and I'll scroll down for you!
radius of a circle  diameter of a circle  circumference of a circle 
area of a circle  chord of a circle  arc of a circle 
arc length of a circle  sector of a circle  area of a sector of a circle 
The radius of a circle:  
The radius of a circle is the distance from the center of the circle to the outside edge.  
The diameter of a circle:  
The diameter of a circle is longest distance across a circle. (The diameter cuts through the center of the circle. This is what makes it the longest distance.)  
The circumference of a circle (the perimeter of a circle):  
The circumference of a circle is the perimeter  the distance around the outer edge. Circumference = 

The area of a circle:  
Area = 

A chord of a circle:  
A chord of a circle is a line segment that connects one point on the edge of the circle with another point on the circle. (The diameter is a chord  it's just the longest chord!) 