Getting An Accurate Ring Size At Home
A proper jeweler's ring sizer is always the gold standard, but a piece of string and a ruler gets remarkably close when you're careful. The two things that throw off a DIY measurement most are temperature (fingers swell in heat and shrink in cold — measure at normal room temperature, not right after a workout or in winter chill) and time of day (hands are typically smallest in the morning and largest in the evening).
Which Finger, And Which Hand?
Always measure the specific finger the ring is meant for — sizes are not interchangeable finger to finger, and most people's dominant hand runs very slightly larger than the other. For an engagement or wedding ring, measure the ring finger on the hand it will actually be worn on, ideally on a few different days to account for natural daily fluctuation.
When In Doubt, Round Up Slightly
A ring that's a touch loose can be resized down fairly easily and inexpensively by a jeweler. A ring that's too tight to remove comfortably is a bigger problem, especially for wider bands, which fit more snugly than thin ones at the same labeled size because they contact more of the finger.