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 exact finger the ring will be worn on
Use an existing ring that fits and measure its inner diameter
Compare the inner diameter to a ring size chart
If between sizes, choose the larger size
Consider ring width when choosing a size
Check sizing with a jeweler if possible