The best way to find your right ring size is by visiting a local jeweller and ask them to measure your finger. Make sure to have the correct finger sized, as each finger has a (slightly) different size—even the same finger on different hands.
Another factor that will influence your finger size is the temperature. In warmer weather, your finger will be slightly thicker than when it's cooler.
Ring sizes used on this website are based on a ring's diameter, shown in millimetres.