Circles are one of the most fundamental building blocks of our world – just go outside and look around you. Look at the circular sun, or the shapes of ripples after you throw a pebble in the water. And let’s not forget all the man-made circles around us, like balls or wheels. Like it or not, circles are all around us.
The iris and pupil are natural circles 
These glasses sport round lenses 
This penny is a circle 
This bubble is a circle
What, really, is a circle?
A circle is a shape, drawn by connecting all the points equidistant from a point. Huh? Simply put, a circle is all the points a certain distant from the center. Still confused? Try drawing a few circles on your own to better understand.
Radius
As we established earlier, a circle consists of all points an equal distance away from the center. This distance is known as the radius. The radius can be used to help us find out many properties of circles.
Circumference
The circumference, or perimeter, of a circle, is the circle’s border. In order to calculate its length, use the formula 2 x π x radius.
Area
The area of a circle can be calculated with the formula A = π x radius2 (π is a constant, approximately equal to 3.14). Here’s a quick breakdown.
For more information on circles, including facts about diameters, check out this circles fast facts sheet.