You will need:
Thread, ribbon or thin rope. Important: It should not be elastic.
Wrap the thread around your finger so that it does not squeeze the skin, and fix or mark the point of intersection. Now move the resulting “ring” to the widest place - the joint. First of all, the jewelry should pass there. Passes? Great, take a ruler and attach a piece of thread to it. Measure with an accuracy of half a millimeter, it is important. Divide the length by 3.14 to calculate the diameter.
Example:
The length of the circumference (in our case, the thread) is 53.5 mm.
53,5 mm : 3,14 = 17 mm - this is the desired diameter.
Now check the diameter with the data on the picture and find your size: