In the U.S. and Canada, women’s ring sizes range from 3 to 9, and men’s from 8 to 14.
The ring should fit comfortably without slipping off, and it should be easy to take off. Keep in mind that finger size can change slightly, depending on the time of day, and the air temperature (fingers tend to be smaller in the morning and when exposed to cold weather). Take that into account to get an optimal reading and to eliminate errors.
To pick the right size without ruining the surprise, ask a family member for help or determine the size based on a ring your loved one is already wearing.
When ordering a ring from abroad, make sure to compare the U.S. size with the local standards using the international ring size conversion chart.