![image0.jpg](https://www.dummies.com/wp-content/uploads/437212.image0.jpg)
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:![image1.jpg](https://www.dummies.com/wp-content/uploads/437213.image1.jpg)
For example, given a circle with a radius of 5 millimeters, you can figure out the diameter as follows:
![image2.jpg](https://www.dummies.com/wp-content/uploads/437214.image2.jpg)
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:![image3.jpg](https://www.dummies.com/wp-content/uploads/437215.image3.jpg)
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):
![image4.jpg](https://www.dummies.com/wp-content/uploads/437216.image4.jpg)
So given a circle with a radius of 5 mm, you can figure out the approximate circumference:
![image5.jpg](https://www.dummies.com/wp-content/uploads/437217.image5.jpg)
The area of a circle
The formula for the area (A) of a circle also uses π:![image6.jpg](https://www.dummies.com/wp-content/uploads/437218.image6.jpg)
Here's how to use this formula to find the approximate area of a circle with a radius of 5 mm:
![image7.jpg](https://www.dummies.com/wp-content/uploads/437219.image7.jpg)