Eh, okay. I'm kind of a science and math geek here, so this post doesn't make sense to me. A toy's circumference and diameter will feel the same inside of your body. They are two different terms that both describe the same thing...the toy's girth. Circumference is the distance around the toy, diameter is the length of a line passing through the toy. Even if the manufacturer hasn't listed the measurements accurately, they usually list the diameter as equal to the circumference using the formula JR has mentioned.