Unlike other chastity brands, we don't recommend trying to measure for base rings. We've never found a reliable way to get good numbers and when people do measure, they often get results that are way off from what they need.If you have experience with other chastity devices that use circular and flat (not curved) rings, the base ring inner diameter should translate to a similar KINK3D base ring. If you have experience with other types of base rings, or need additional help, feel free to email us.