To find the exact ring size you can use these two methods:

1) Measure the diameter or the internal circumference of your ring, then evaluate the measurement using the table you see below.

2) Print attached PDF document and place one of the rings you have on top of the circles. The precise size of the ring is given by the number where the inner circumference of the ring is perfectly adjacent to that of the circle (the circumference of the circle must be visible).