MEASURE YOUR FINGER
To find your size, first, wrap the measuring tape (or string) around the base of the finger on which you plan to wear your ring. You want the string or tape to be snug, but not tight. Ideally, the measuring tape should be cloth — if you are struggling to wrap a measuring tape around your finger, just switch to a string to ensure your measurement is accurate.
Use a pen to mark the tape where it overlaps or cut the small piece of string with scissors. Then, simply measure the string in millimeters, or note how many millimeters it took to wrap the measuring tape around your finger. The number you'll get is the European size.
MEASURE YOUR RING
If you have a ring that already fits your finger well, you can use a ruler to measure the inside diameter of this ring. Take your ruler and measure straight across the inside of the ring from edge to edge - you'll get the inside ring diameter in millimetres. Then use a conversion table above to get a European size.