Ring size

Ring size is a measurement used to denote the circumference (or sometimes the diameter) of jewellery rings and smart rings.

Measuring tools
Ring sizes can be measured physically by a paper, plastic, or metal ring sizer (as a gauge) or by measuring the inner diameter of a ring that already fits.

Ring sticks are tools used to measure the inner size of a ring, and are typically made from plastic, delrin, wood, aluminium, or of multiple materials. Digital ring sticks can be used for highly accurate measurements.

International standard
ISO 8653:2016 defines standard ring sizes in terms of the inner circumference of the ring measured in millimetres. ISO sizes are used in Austria, France, Belgium, Scandinavia (Norway, Sweden, Denmark, Finland, Iceland), and other countries in Continental Europe. This international standard specifies a method to measure the ring-size using a ring stick (also called a mandrel or triblet) with defined characteristics, which is used during manufacturing steps, and specifies the designation of the ring-size.

For jeweller-consumer relationships, the finger size is measured with a finger gauge set made up of a ring for each size with the same diameter and tolerance as the ring stick ones. The sizes are in millimeters and correspond directly to the outer circumference of the ring stick to the inner circumference of the finger gauge.

Other traditional and regional systems
Other ring size measurement systems are used in areas that do not use ISO 8653:2016.

North America
In the United States, Canada, and Mexico, ring sizes are specified using a numerical scale with $1/4$ steps, where whole sizes differ by 0.032 in of internal diameter, equivalent to undefined in of internal circumference. The relationship of this size ($$s_{NA}$$) to ISO 8653:2016 circumference ($$C_{ISO}$$) is $$C_{ISO} = 2.55 \times s_{NA} + 36.5$$, while the relationship to ISO 8653:2016 diameter ($$d_{ISO}$$) is $$d_{ISO} = 0.8128 \times s_{NA} + 11.63$$.

The Circular of the Bureau of Standards summarizes the situation with this system: "While there apparently is only one standard in use in the United States, in reality, because of the lack of specific dimensions and because of the errors introduced by the adoption of a common commercial article as a pattern, there are many, although similar, standards." The standards are generally consistent and remain so. There does not appear to have been any improvement in the standard since then.

Ireland, United Kingdom, Australia
In Ireland, the United Kingdom and Australia, ring sizes are specified using an alphabetical scale with half sizes. Originally in 1945, the divisions were based on the ring inside diameter in steps of 1/64 in. However, in 1987 BSI updated the standard to the metric system so that one alphabetical size division equals 1.25 mm of circumferential length. For a baseline, ring size C has a circumference of 40 mm.

India, Japan, China
In India, Japan and China, ring sizes are specified using a numerical scale with whole sizes that do not have a linear correlation with diameter or circumference.

Germany and Netherlands
Netherlands, Germany, and sometimes Argentina use a standard (referred to as the German System  ) where ring sizes are defined by the diameter of the ring, measured in mm. This system may also be used at times in Russia.

Italy, Spain, Switzerland
In Italy, Spain, and Switzerland, ring sizes are specified as the circumference minus 40 mm: for example, size 10 in this system is equivalent to ISO 8653:2016 size 50. This may also be referred to as the Swiss Ring Size System.

Russia
In Russia, ring sizes are equal to the inner diameter rounded to whole and half numbers, sometimes to quarters, for example diameter 16.92 mm is equal to size 17, 16.1 mm is equal to size 16.

Resizing
Most rings can be resized; the method of doing so depends on the complexity of the ring and its material. Rings of soft material may be enlarged using mechanical stretching. For example, the ring may be enlarged using a rolling mill, a steel ring mandrel, or a Schwann Ring Stretcher.

Adding Material
In some cases, the ring may need to be cut open and material either added or removed before fusing the ring together again. The ring may be slightly heated to reveal any solder line so the jeweler can open the ring on the same seam so as to minimize the total number of solder joins on the ring.

Sizing beads
Small metal beads called sizing beads can be added to the inner circumference of a ring to:
 * Decrease the effective inner diameter of a ring that is too big, to aid in holding the ring in place against the finger
 * Counterbalance top-heavy rings
 * Keep a ring from spinning for wearers whose knuckles are much larger than their finger base

Sizing beads are typically made of the same metal as the rest of the ring since it is easier to solder two similar metals.