Establish the diameter of the circle you will be using in your calculations. If you know the circle's radius, multiply it by 2 to get the diameter.
Multiply the circle's diameter times pi, which is approximately 3.14159. You can use 3.14, as well, for the sake of brevity. The answer is the circle's circumference.
For example, if a diameter is 6 inches, 6 x 3.14 = 18.84. The circumference of the circle is 18.84 inches.
Check your work by dividing the answer by pi. If you arrive at the original diameter you have properly done your calculations.