Follow these instructions to determine which size cockring from you should purchase. 1. Take a string and wrap it around/ behind your balls and the base of your flaccid penis. 2. Mark the string on both ends where they come together. Donít pull the string tight, but rather just encircling your privates. Remember that a cock ring is rigid and therefore wonít expand or contract when worn. 3. Remove the string and measure between the marks. This measurement will give you your circumference. Divide the circumference by pi (3.14). The result is the diameter. Pick the closest diameter from the standard sizes below. standard cock ring sizes: Circumference Diameter (ring size) 5.50" 1-3/4" 5.89" 1-7/8" 6.28" 2" 6.67" 2-1/8"