Wrap a strip of paper or 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 more than once for accuracy
Make sure the ring can pass over the knuckle
Use an existing ring that fits well
Measure the inside diameter of that ring
Compare the diameter to a ring size chart
Ask a jeweler to measure the ring size
Consider the width of the ring when choosing the size