Hello, I'm selling my Oura ring after learning a lot from it over the last year. Bought it $500 (with tax and import fees). The ring is actually brand new, as the battery was losing power, and when I contacted Oura about getting the battery replaced, they sent be a brand-new ring! Which is the one I’m selling. Be aware that to use all the features of the Oura App, you have to paid a monthly subscription of $10. The inner diameter is 21 mm exactly. Feel free to reach out if you have any questions. Thanks, Stéphane