You can determine the correct inner circumference of the ring with a thread or a piece of paper. Wrap the thread or paper tightly around the part of the finger you want to buy the ring for and measure the length. The inner circumference of the ring is then the length in millimetres.
Please also note the following:
The fingers of the right and left hand are usually different in thickness, so measure exactly the finger on which you want to wear the ring.Your ring size changes throughout the day and depends on the temperature. Therefore, your fingers are often thicker in the evening than in the morning and usually thicker in summer than in winter. In the course of your life, your ring size will also change. Therefore, do not fall back on a size determined earlier.
The ring sizes are according to the following inner diameters in millimetres: