Circle
Pronunciation: /ˈsɜr kəl/ ?
Figure 1  Using a compass to draw a circle. 

A circle is all points in a plane that are
equidistant
from the
center
point.^{[1]} When using a
compass
to draw a circle, the point of the compass is the center of the circle, and
the stylus marks all points that are the same distance from the center.

Properties of a Circle
Click on the blue points in manipulative 1 and drag them to change the figure. Double click on
the manipulative to open it in a fullscreen window.
 Manipulative 1: Properties of a circle. Created with GeoGebra. 

 The center of a circle
is the point from which all points of the circle are equidistant.
 A radius of a circle is a line segment from the center of the circle to one of the points on the circle.
 A diameter of a circle is line segment from one point on the circle to the opposite side through the center of the circle. The length of a diameter is twice the length of a radius (d = 2r).
 The circumference of a circle is the edge of the circle. Circumference can also refer to the length of the edge of the circle.
 A chord of a circle is a line segment from any point on the circle to any other point on the circle. See Chord.
 An arc is a portion of the circumference of the circle.
 A sector is a portion of a circle between two rays radiating out from the center of the circle.
 A segment is a portion of a circle cut off by a chord of the circle.
 A tangent of a circle is a line that touches the circle at exactly one point. The point where the tangent line touches the circle is called the point of tangency.
 A tangent segment of a circle is a segment of a line tangent to a circle extending from the point of tangency to a point of intersection with a ray extended from the center of the circle.

Understanding Check
 Is a diameter also a chord? Try moving the endpoints of the chord in figure 2 on top
of the endpoints of the diameter.
Yes.
Correct. Since a chord is a line between any two points on the circle, and a diameter is a line between a point and a point opposite that point, a diameter is a special case of a chord.
No.
Incorrect. Since a chord is a line between any two points on the circle, and a diameter is a line between a point and a point opposite that point, a diameter is a special case of a chord.
 Does each circle have exactly one diameter? Try moving the endpoints of the diameter in figure 2.
Yes.
Incorrect. Each circle has an infinite number of points on its circumference, it also has an infinite number of diameters. However, all of the diameters of a circle have the same measure.
No.
Correct. Each circle has an infinite number of points on its circumference, it also has an infinite number of diameters. However, all of the diameters of a circle have the same measure.
 What is the relationship between the measure of a radius and the measure of a diameter of a circle?
The measure of a radius has no relationship to the measure of a diameter.
Incorrect. A radius goes halfway across a circle. A diameter goes all the way across a circle. So the measure of a radius is 1/2 the measure of a diameter.
The measure of a radius is 1/4 the measure of a diameter.
Incorrect. A radius goes halfway across a circle. A diameter goes all the way across a circle. So the measure of a radius is 1/2 the measure of a diameter.
The measure of a radius is 1/2 the measure of a diameter.
Correct. A radius goes halfway across a circle. A diameter goes all the way across a circle. So the measure of a radius is 1/2 the measure of a diameter.
The measure of a radius is exactly the same as the measure of a diameter.
Incorrect. A radius goes halfway across a circle. A diameter goes all the way across a circle. So the measure of a radius is 1/2 the measure of a diameter.
Definitions of Variables 
Variable  Represents 
r  radius 
d  diameter 
c  circumference 
a  area 
h  xcoordinate of the center of a circle 
k  ycoordinate of the center of a circle 
Description  Equation 
Diameter of a circle  d = 2r 
Circumference of a circle  c = 2πr 
Area of a circle  a = πr^{2} 
Equation for a circle in Cartesian coordinates  (x  h)^{2} + (y  k)^{2} = r^{2} 
Equation of a circle with center at the origin in Polar coordinates  r = r_{0} 
Table 1: Formulas relating to a circle 
CenterRadius Equation of a Circle
 Manipulative 2: Centerradius equation of a circle. Created with GeoGebra. 

The centerradius equation of a circle is
(x  h)^{2} + (y  k)^{2} = r^{2}. In this equation
the point (h,k) is the coordinates of the center of the circle, and r
is the radius of the circle.
Manipulative 2 allows you to explore this equation for a circle. Click on the points on
the sliders in manipulative 2 and drag them to change the figure.

Plotting a Circle
For this demonstration use the centerradius equation for a circle centered at (1,2):
(x + 1)^{2} + (y  2)^{2} = (1.5)^{2}.
Step  Diagram  Description 
1 

Since x  h = x + 1, h = 1.
Since y  k = y  2, k = 2.
So the center of the circle is at the point (1,2).
Plot the point (1,2). 
2 

Now draw the two points that are on the circle to the right and left of the
center. Since the radius is 1.5, the left point
will be at (1  1.5, 2) = (2.5, 2) (the xcoordinate of the center
of the circle minus the radius). The right point will be at
(1 + 1.5, 2) = (0.5,2) (the ycoordinate of the
center of the circle plus the radius). 
3 

Now draw the two points that are on the circle that are on the top and bottom of the
center. Since the radius is 1.5, the top point
will be at (1, 2 + 1.5) = (1, 3.5) (the
ycoordinate of the center
of the circle plus the radius). The bottom point will be at
(1, 2  1.5) = (1,0.5) (the ycoordinate of the
center of the circle minus the radius). 
4 

Now draw a circle through the four points. 
More Information
 McAdams, David. Conic Section. allmathwords.org. Life is a Story Problem LLC. 20090312.
 McAdams, David. Center of a Circle. allmathwords.org. Life is a Story Problem LLC. 20090312.
 McAdams, David. Pi. allmathwords.org. Life is a Story Problem LLC. 20090312.
Educator Resources
Cite this article as:
Circle. 20100127. All Math Words Encyclopedia. Life is a Story Problem LLC. http://www.allmathwords.org/en/c/circle.html.
Translations
Image Credits
Revision History
20100127: Expanded parts of a circle to include sector, segment, tangent, point of tangency, and tangent segment. Expanded verbiage on centerradius equation of a circle. (
McAdams, David.)
20091228: Added "References" (
McAdams, David.)
20081125: Added link to chord.html in 'Parts of a Circle' (
McAdams, David.)
20081027: Changed manipulative from geometer's sketchpad to geogebra. Corrected formula for the area of a circle. Added manipulative for the equation of a circle (
McAdams, David.)
20080428: Added section on plotting a circle. Added conic section to more info (
McAdams, David.)
20070712: Initial version (
McAdams, David.)