The best way to get the right fit is to have your beloved visit the jeweler with you and have the jeweler determine her ring size. A jeweler will probably use a set of finger gauges, often called a ring sizer, which contains a series of metal bands in ½ size increments that slide onto the finger to measure it for the most secure fit and best comfort. Your jeweler will also know how to factor in temperature, humidity, altitude and even time of day, which can also affect finger size. This takes any uncertainty and guessing out of the process.
If you want to keep the element of surprise, you can always “borrow” a ring from her jewelry box and take it to a jeweler yourself. Just make sure it’s a ring she often wears on a ring finger as opposed to one she wears on her middle finger, thumb or index finger, as these will not be the same size. In addition, for a person who is right-handed, the left ring finger is typically smaller than the right ring finger by about half a size and vice versa. Let the jeweler know if the ring you’ve brought is usually worn on the right or left hand and which hand is dominant.
To measure an existing ring, the jeweler will typically use a ring mandrel – a tapered cylindrical tool – that is marked with ring sizes. As with a finger gauge set, it can measure in half as well as full sizes. If the band is unusually wide, take the measurement from the center.
If none of these options work for you, some online merchants offer plastic ring sizers. These can give a fairly accurate measurement of her ring size.