Take a piece of string and wrap it around where you want the ring to fit, pull it snug, make a magic marker dot on the string anywhere two turns are side by side. Unwrap the string, measure the distance between the marks, divide the number by 3.14 and you have the diameter you need. I'd suggest getting one that size and other about 1/4" (0.5 cm) larger and smaller - to see which you prefer.