If you already own a ring in your size, you can find the size engraved inside your ring. If the size is not visible, print the ring finder at full scale, then place your ring on the circle that bets fits the inside of the ring. That is your size.
If you prefer not to print, measure the internal diameter of your ring and use the chart below to determine your size. For example if the internal diameter of your ring is 16.24 mm, it is a ring of size 51.