The fingers of the left and right hand are often different in width. Please calculate your ring size at the hand where you will wear the ring.
Fingers are usually thinner in winter than in summer. The lower the temperature, the thinner the fingers.
The size of your finger changes over the course of your life, so please do not rely on ring sizes measured long time ago. Determine your ring size as much as possible directly before you buy.
The size of the finger also changes often during the day. So the fingers in the morning are usually thinner than in the evening.