The inner ring circumference is always used for the ring size.
You can measure the correct inner ring circumference by using a thread or a piece of paper. Wrap the thread or paper tightly around the finger you want to wear the ring on and measure the length. The inner ring circumference is the length in millimetres.
Alternatively, you can download a ring template here.
Measure the inner circumference of a ring you already have and compare it with the measurements in the template. The ring sizes correspond to the subsequent inner circumferences in millimetres.