There are a few quick and easy steps to choose the right size for you. The tools you need are a ruler or a dressmaker’s tape measure and a ring that you already have. You have to place the ruler on a linear support surface and place the ring perfectly centred as in the photo and the two ends must coincide with the lines designating the millimetres. Then, to calculate its diameter, it is sufficient to count how many rods, i.e. how many mm, are included within it. Finally, you only have to compare with the table, our advice as specified above is to round to the nearest whole millimetre, and choose the size of that diameter.