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.
To find your ring size: Simply measure the circumference (distance around) your finger (with something flexible, such as a strip of paper or piece of string, and a ruler with millimetre measurements). Or, measure the diameter (distance across) in millimetre of a ring you currently wear. Find the nearest measurement on the chart below. That's approximately your ring size. (When in doubt between two sizes, it is always safer to go to the next larger ring size. This ensures your new ring will fit you.)