Our dominant hands are generally slightly larger — make sure you measure the right finger on the correct hand!
Rings with wider bands will fit a bit tighter. Measure a ring with a with a similar width as the ring you want to purchase.
Make sure the finger is at normal body temperature — fingers can shrink or expand when hot or cold.
Your fingers tend to change size throughout the day — the best time to measure your finger is the end of the day.
If your knuckle is bigger, take 2 measurements and choose a size in between — ring needs to fit over knuckle but not be too loose or it will shift around.