Cliff, welcome to the forum. I will attempt to answer and hope to be some help. Let's go back to math class and see if that will help. C=piRsquared. Does that sound familar? Yea it does to me too, but darn if I want to do it that way. An easier way for me would be to stand a tape measure on it's side and actually measure around. But if you are good at math, the above formula may be easier for you.
One trick I learned many years ago for sewing a band on a circle was to first fold the circle in half and then half again to find the quarters. Mark those with a pin or some method. Do the same with the band, folding twice to find quarters. One thing to note though, be sure to add seam allowance to the band and when you fold the second time only fold to the seam allowance. You can sew the seam first and use the seam as one of the quarter marks.
Now match your marks and pin the band to the circle. At this point, I will pin again in the opening at that center. Continue in this fashion until the band is secure enough to sew. After you have sewn the band to the circle, press the seam allowance in the direction it wants to go, which would probably be towards the circle.
Let me write out the formula for you, as I do not have math symbols on my keyboard.
Circumferance= pi x Radius squared