The size of your fingers can vary depending on the time of day. To ensure a reliable result, do not measure your fingers when your hands are cold, and take measurements, if possible, at the end of the day.
Measurements can vary slightly between the two hands. A ring worn on the left hand will not necessarily be the exact same size as one worn on the same finger of the right hand.
Ensure that the size chosen offers sufficient room for the ring to slide easily along the entire finger.