Select a ring you already own that fits the desired finger, or cut a small piece of thread.
Use a ruler or tape measure and place the ring on the top or cut thread, measuring the millimeters from side
to side of the ring or cut thread. in the case with ring measured you get a diameter, in the case of cut thread you get a length of circumference, you should know the diameter of the inner circle. Compare these measurements with size guide in millimeters.
Finger size can also slightly vary throughout the day or depending on the season, your fingers tend to swell in warmer weather or after physical activity, and be a bit slimmer in cooler temperatures or early in the day.