Geometry Tutorials: How to calculate the surface area of a circle

To calculate the surface area of a circle two separate elements are required. The first is the circle's radius. In case you did not pay much attention at school the radius is half of the circle's diameter. The second element that we will require is Pi (Π):

Π = 3.1415926535897932384626433...

And on and on it goes. Most maths teachers are happy enough when you simply use 3.14159. If you are asking your self what's the big deal about Pi, it gives us the relationship of a circle's circumference to its diameter. Because this relationship is consistent it doesn't matter if the circle has a diameter of 10cm or 10km. Pi will stay the same. Here then is how it can be used to calculate a circle's surface area:

A = Πr²

So, the surface area (A) is equal to Pi times Radius Squared. That's simple enough! All that you need to do now is add the correct units of measurement to the result and you have accomplished your goal. For instance, if you are using a diameter of cm, your result will be in cm squared.