A ring should feel secure without pinching or making your finger uncomfortable. Fingers can vary in size with temperature and other everyday conditions, so a measurement taken at one moment is only a guide. Measure the finger where you plan to wear the ring, and use the sizing method provided by the maker if one is available.
Compare the ring's stated size with a ring that already fits well on the same finger. Remember that wider bands can feel snugger than narrow ones, even when the marked size is the same. If you are between sizes, check the seller's fit guidance and return policy before ordering rather than guessing based on a different finger.
When trying on a ring, it should pass over the knuckle with gentle resistance and sit comfortably once in place. It should not leave deep marks or spin constantly. Adjustable rings can offer flexibility, but frequent bending may stress some designs, so follow the maker's instructions. A comfortable fit is more important than choosing a size because it is usually yours.