Wrap a strip of paper or a piece of string around the base of the finger
Mark where the ends meet
Measure the length with a ruler in millimeters
Compare the measurement to a ring size chart
Measure the finger at the end of the day
Measure the finger when it is warm
Measure the exact finger the ring will be worn on
Measure the inside diameter of an existing ring that fits well
Match the diameter to a ring size chart
Repeat the measurement to confirm accuracy