Question: how to measure ring size in cm diameter?
Answer: Use the string and tie it slightly loose around the finger that you wish to wear the ring on. ...  Finger Size 5 = Finger's measurement 5cm. Finger Size 6 = Finger's measurement 5.5 cm. Finger Size 7 = Finger's measurement 6 cm. Finger Size 8 = Finger's measurement 6.5 cm. Finger Size 9 = Finger's measurement 7 cm.