The ring sizer should fit your finger comfortably. It needs to be tight enough so that it won't fall off, but, at the same time, loose enough to slide over your knuckle.
Also bear in mind that one's finger size can vary depending on the time of day, the weather and other temperature changes. The best result is obtained when you measure your finger size during the day. Fingers tend to be smaller when cold (in winter) and tend to be swollen when warm (in summer, in the early morning, after wearing gloves,..).
To eliminate an erroneous reading, it is best to measure your finger size up to 3 or 4 times.
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