Ring size is not the same as finger size. A professional jeweler can determine proper ring size using a set of steel gauge-rings. These "ring sizers" come in narrow and wide widths. Fingers change over the course of the day due to diet or work affecting the results using gauge rings.
Other methods, such as measuring the finger with a strip of paper, or slipping the finger through pre-cut "measuring holes", are inaccurate. It has been recommended that fingers be measured at the end of the day, when they are at their largest diameters. Existing fitted rings can also be measured for increased accuracy. Other factors that can affect ring size include: the menstrual cycle, temperature, humidity, water-retention and exercise.
The best strategy for measurement is to purchase calipers that measure inside diameters. A tightly-calipered measurement should then be taken of various rings that fit; these should be noted and the corresponding size recorded. The MANDREL, the tapered ring-measuring rod, should be regarded as only half-accurate. Different jewelers will employ the mandrel in different ways. For instance, one jeweler may use the mandrel and take an edge-of-the-ring reading; others will take the measure that corresponds to the largest finger the ring can fit.
It is best for the consumer to know and be firm about finger dimensions, because every jeweler or ring seller will think his/her method the "best" ring-measuring method. It has often been suggested that the entire world switch to using inner diameter (the diameter of the finger) as the only size-number standard.
A wider band can be worn more loosely. Large knuckles/hands should wear a slightly loose ring, while slimmer knuckles/smaller hands will require a tighter ring. Some rings can be re-sized, but many cannot. Rings made of titanium or tungsten steel, stone-set rings with complicated or precise mountings, extremely thin/wide rings cannot be re-sized.
A good way for a "rough-and-ready" measurement is to find a ring that is just smaller than the second knuckle of the finger. This way the fit is comfortable but will not slide off the finger. The method of trying on a ring and shaking the hand violently, to see if the ring comes off, is inadvisable and not very accurate.
Furthermore, every ring size chart will vary slightly in its standards. This always occurs unless one company has copied another company's size chart. Even the ISO, ringsmiths and jewelers use different sizing standards, assigning different numbers to the same measurements. On one chart a diameter of 20.65mm may equal a size 10 3/4; on another chart, a similar number, such as 20.62mm, may read out as size 10. It is wise to keep all of the above firmly in mind when ring shopping.
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If ordering from catalog or internet, always demand that the seller provide the inner diameter of a ring.