Use a soft measuring tape or a strip of paper
Wrap it around the base of the finger where the ring will sit
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 tape or paper is snug, not tight
If the knuckle is larger, measure both the base and knuckle
Choose a size that can slide over the knuckle comfortably
Measure the exact finger for the hand and finger you will wear the ring on
Avoid measuring when your fingers are cold or swollen