The sizing for all 3cap rings is essentially based on the ring-finger size.
Due to its open-band design and slight adjustable feature, the ring is able to fit anywhere between a full size range.
I always recommend to select a size slightly under your actual ring size. It is much better (and looks nicer!) to adjust the ring to the perfect fit by gently opening the extremities apart, than it is to close them together for a tighter fit.
Once you have found the "sweet spot" for the ring to be put on and taken off easily, you should try to maintain the openings as-is and avoid further bending action. Doing so will greatly extend the life of your ring :)
::: If you already know the US size for your ring-finger, please refer to the tables below so you can determine your '3cap size":
::: If you need to find out your ring size first, the following links can be helpful:
• International Ring Size Chart: http://www.ringsizes.co/index.html
• Ring Size Calculator: https://www.omnicalculator.com/everyday-life/ring-size
::: For all other size requests, please email me at firstname.lastname@example.org ...I'll be happy to help you!