*center*of a circle is a point that's the same distance from any point on the circle itself. This distance is called the

*radius*of the circle, or

*r*for short. And, any line segment from one point on the circle through the center to another point on the circle is called a

*diameter*, or

*d*for short.

## The diameter

As you can see, the diameter of any circle is made up of one radius plus another radius — that is, two*radii*(pronounced

*ray*-dee-eye). This concept gives you the following handy formula:

For example, given a circle with a radius of 5 millimeters, you can figure out the diameter as follows:

## The circumference

Because the circle is an extra-special shape, its perimeter (the length of its "sides") has an extra-special name: the*circumference*(

*C*for short). Early mathematicians went to a lot of trouble to figure out how to measure the circumference of a circle. Here's the formula they hit upon:

** Note:** Because 2 x

*r*is the same as the diameter, you also can write the formula as

*C*= π x

*d*.

The symbol π is called *pi* (pronounced "pie"). It's just a number whose approximate value is as follows (the decimal part of pi goes on forever, so you can't get an exact value for pi):

So given a circle with a radius of 5 mm, you can figure out the approximate circumference:

## The area of a circle

The formula for the area*(A)*of a circle also uses π:

Here's how to use this formula to find the approximate area of a circle with a radius of 5 mm: