In this module, you can study how to calculate the distance around a circle (its circumference) if you know its diameter.
The above video from Khan Academy provides an excellent introduction to understanding the radius, diameter and circumference measures of a circle.
The circumference of a circle is not equal to the length of two diameters.
A circle's circumference divided by its diameter is the same for every circle. (It's pi, or 3.14)
Study and practice
Use these search terms to find additional resources:
how to find circumference