Wrap a strip of paper or a flexible measuring tape 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 strip fits snugly but not tightly
Measure the same finger on the hand where the ring will be worn
If between sizes, choose the larger size
Use an existing ring and measure its inner diameter if available