To find the right size for you, follow the method below. If you're shopping for someone else, the best way to find their ring / wrist size is to ask them or if you're planning a surprise, get help from friends and family.
How To Measure Ring / Wrist Size with String, Paper & Ruler
Wrap string or paper around the base of your finger / around your wrist.
Mark the point where the ends meet with a pen.
Measure the gap between the points against a ruler
Measure the string or paper with a ruler (mm) to get the circumference then divide by 3.14 (or Pi) to get the diameter.
Pick the closest measurement on the ring size chart to find your ring size.
Measure your ring size at the end of the day when your fingers are warm. Finger size changes depending on the time of day and the weather, and your fingers are actually smaller in the early morning and in cold weather. Also, keep in mind that fingers on your dominant hand are usually larger.