Before buying a ring it’s important to determinate the size needed for a perfect fitting ring. Typically there are two ways in which you can measure the size.
The first one consists in placing any of your rings in the following chart. The internal part of the ring should be aligned with the external edge of the circle, if your ring is between two sizes is recommend to buy the biggest one as the measures of the finger could vary depending on the temperature.
The second way to know your exactly size is to cut a thin strip of paper, then put it around the finger, it should be below of the joint of the finger and close to the knuckle . Once this is done you only need to mark the point where the paper fits and finally, measure the distance.