The circumference of a circle is its "perimeter," or the distance around its edge. If we broke the circle and bent it into one flat line, the length of this line would be its circumference:

Circumference of a Circle

The diameter of a circle is a line segment from one point on the edge of the circle to another point on the edge, passing through the center of the circle. It is the longest line segment that cuts across the circle from one point to another. There are many different diameters, but they all have the same length:

Diameters of a Circle

The radius of a circle is a line segment from the center of the circle to a
point on the edge of the circle. It is half of a diameter, and thus its length
is half the length of the diameter. Again, there are many radii, but they all
have the same length. In the following diagram,
*a*
,
*b*
, and
*c*
are all radii:

Radii of a Circle

The area of a circle is the total number of square units that fill the circle. The area of the following circle is about 13 units. Note that we count fractional units inside the circle as well as whole units.

Area of a Circle

Mathematicians have discovered a special number, called
pi (represented by
*Π*
),
which is the ratio of the circumference of *any* circle to the length of its diameter.
*Π*
is roughly equal to 3.14--most scientific calculators have a "
*Π*
" button that
will produce more digits.
*Π*
is a non-
terminating, non-
repeating decimal; thus,
*Π*
is an
irrational number.

Since
*Π*
is the ratio of the circumference to the diameter,
*Π* = *c*/*d*
;
*c* = *Π*×*d*
; and
*d* = *c*/*Π*
; where
*c*
and
*d*
are the
circumference and the diameter, respectively. The most important equations to
remember are the last two.