You will need
  • Paper, pen, calculator
First we find the diameter D (diameter is a straight line which connects the two most remote from each other point of the circle always passes through the center). This will double the radius of the circle r: D=2r.
Now you have all the data in order to find the length of circumference L. Use the formula L=?D. Number ? is the ratio of a circle's circumference to its diameter, it is the same for all circles and is approximately equal to 3.14. Thus, to calculate circumference, multiply 3.14 by the diameter..
Knowing the radius, you can also calculate the area of a circle by multiplying the number ? (3.14) times the radius squared: S = ?R2.