Wrap a strip of paper or string around the base of the finger
Mark where the ends meet
Measure the length in millimeters
Compare the measurement to a ring size chart
Measure the finger at the end of the day
Measure more than once for accuracy
Make sure the paper or string is snug, not tight
Measure the same finger on the hand you will wear the ring on
If between sizes, choose the larger size
Use an existing ring that fits and measure its inner diameter
Match the inner diameter to a ring size chart
Visit a jeweler for a professional sizing