Remember, when taking an AC measurement, never lay the tape on the body. Measure Tip to Tip from the back side. Size in "cm" convert to "inches". 1 inch = 2,54 cm. Example: The size of your shoulders is 46cm (46,00 : 2,54 = 18,11" - Shoulder pad size is Medium)
If you are between two sizes, buy smaller ones. Use the size chart of the relevant product and manufacturer.
Shoulder pads sizing chart: