For bands of 8 mm and wider the allowance grows: jewellers move up a quarter to a half size, more if the knuckle is prominent, because a broad band sits flat and grips.
ISO to US conversion
An ISO size is the inner circumference in millimetres, and turning it into a US number divides that growth into the US scale's fractional-inch steps. ISO 54 is US 7 and ISO 62 is US 10, but because the units differ, no single constant converts the two and the tables read row by row. An ISO value between two US sizes rounds to the nearest, so a 55 mm finger sits between US 7 and 7.5, where the wider band should take the larger.
At this size, ISO to US
One size up from ISO 53, US moves 0.5 to US 7 while ISO rises 1 to ISO 54, so the two scales spread wider. ISO 53 is whole but US 6.5 carries a half, so a rack of full sizes only rounds to the nearest. For ISO 53, the US column here runs US 6, US 6.5, US 7, stepping a half size at a time.
Ring Sizes Around ISO 53 US UK ISO / EU JP / KR Diameter (mm) Circumference (mm) 5 J 49 9 15.7 49.3 5.5 K 51 10 16.1 50.6 6 L 52 12 16.5 51.9 6.5 M 53 13 16.9 53.1 7 N 54 14 17.3 54.4 7.5 O 56 15 17.7 55.7 8 P 57 16 18.1 57
Frequently Asked Questions Does ISO 53 round up or down in US? For a band wider than about 6 mm, round up: jewellers add roughly a half size to keep a broad ring from binding, and that room cannot be added afterwards.
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Note ISO 8653:2016 sets the size as inner circumference in mm; France and Germany label by the same number, so a ring bought anywhere in Continental Europe needs no conversion. Japan and Korea share one scale: 1 gou is a 13.0 mm bore and each step adds a third of a millimetre, so US 7 (17.3 mm) is 14 gou and 14 ho. Russia and the CIS size by inner diameter in mm to the nearest 0.5, so US 7 is a 17.5 there. A band wider than 6 mm is usually sized up about half a size, and a finger can swing close to a full size between a cold morning and a hot afternoon, so measure warm and late in the day. See a number that looks off? Report an error on this page.