Circles

A circle is a shape on a plane in which all points are equally distant from the center.

Terms and Symbols Related to Circles

Term Symbol Definition
Area $a$ The amount of space occupied by the surface of the circle
Circumference $c$ The perimeter of a circle
Diameter $d$ The distance of a straight line from one point on a circle to the opposite point on the circle and passes through the center
Pi $\pi$ The ratio of a circle's circumference to its diameter. Equal to about $22/7$ or $3.14$
Radius $r$ The distance from the center to a point on the edge of a circle

Formulas to Find Properties of Circles

Area of a Circle

$a = \pi r^2$

$a = \pi (d/2)^2$

Circumference of a Circle

$c = 2 \pi r$

$c = d \pi$

Diameter of a Circle

$d = 2r$

$d = c/\pi$

$r = d / 2$

$r = c/2\pi$