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.