Choosing the right ring size is important, here are two techniques to measure yours.
Take a well-fitting ring and measure the diameter of the inside of your ring.
Use a measuring tape to measure the circumference of your finger. If you don't have a measuring tape, use a piece of paper or string, no thicker than 0.5 mm, and then measure the length of the paper / string with a ruler. This is the circumference of your finger.
Use the following ring size converter chart below to determine your ring size. Look at circumference mm.