Depending on manufacturer two parameters are used in Europe for rings, the circumference of the finger (inner circumference of the ring, e.g. 50, 52, 54) and inner diameter (e.g. 17, 18, 19). Hence we see that the finger circumference is much more precise than the inner diameter.
The simplest method to determine the size of your ring is to measure a ring which fits. However, if this is not possible you can use a piece of string to measure the circumference of your finger at the place where you wish to wear a ring. Alternatively, you can also wrap a strip of paper closely round the knuckle, making marks where the two ends up the strip meet and then measure the difference the distance between these two marks.
Please remember that the circumference of your finger and hence the required size of the ring changes with time and temperature. Fingers are normally thicker in the evening and at high temperatures. Furthermore your ring size changes throughout your life. For this reason you should certainly avoid specifying a size determined some time ago.