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:
