Wrap a strip of paper or string around the base of the finger
Mark where the ends meet
Measure the length in millimeters with a ruler
Compare the measurement to a ring size chart
Use a ring that already fits
Measure the inside diameter of the ring in millimeters
Match the diameter to a ring size chart
Measure the finger at the end of the day
Measure the finger at normal room temperature
Measure the exact finger the ring will be worn on
Repeat the measurement 2 to 3 times for accuracy
If between sizes, choose the larger size