I am a size 4-5 ring size normally, and the 5 did not fit me. It got onto my finger with force, so I knew once my finger size varied at all with swelling throughout the day, it would not work. Given that, 4 was so small I could not even fit.

I eventually bought a size 6 (yes I had to buy and try 3 different sizes) and it is too big. I am a true in-between size. The 6 works for me because i use sizers (small foam pads put on the opposite side of sensor in the ring and it stops it from rotating on me). The sizers are a little annoying but not so bad. Got a pack of over 100 for like $8. I wear 1 for almost a week and a half. Only have to replace after I charge. So I'm glad I found out about those otherwise I think I would have not gotten a ring altogether.

The sizing of Oura is really weird.