Your ring should fit your finger comfortably; snug enough so that it will not fall off, but loose enough to slide over your knuckle.
Finger size changes depending on the time of day and the weather. For best results measure your finger size:
At the end of the day and when your fingers are warm (Fingers are smaller in the early morning and when cold.)
Measure finger size 3 to 4 times to eliminate an erroneous reading.
Women’s most common ring sizes: 5 through 8.
Women’s average ring size: 7.
Men’s most common ring sizes: 8 through 12.
Men’s average ring size: 10.
Ask your significant other’s friends or mother if they know her finger size.
Borrow one of your significant other’s rings (from the correct finger) and use the ring measurements above to determine its size.