Each ISO size is pinned to a diameter within 0.02 mm: 65 here means a 20.6 mm bore, checked on a calibrated steel ring stick 160 mm long.
Brand fit: Precious-metal bands in gold or silver leave this US 11 (64.6mm) resizable, so the 20.6mm bore can be adjusted after the finger changes.
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 65, US moves 0.5 to US 11.5 while ISO rises 1 to ISO 66, so the two scales spread wider. Both ISO 65 and US 11 are whole numbers here, so nothing rounds. For ISO 65, the US column here runs US 10.5, US 11, US 11.5, stepping a half size at a time.
Ring Sizes Around ISO 65 US UK ISO / EU JP / KR Diameter (mm) Circumference (mm) 9.5 S 61 20 19.4 60.8 10 T 62 21 19.8 62.1 10.5 U 63 23 20.2 63.4 11 V 65 24 20.6 64.6 11.5 W 66 25 21 65.9 12 X 67 26 21.4 67.2 12.5 Y 68 27 21.8 68.5
Frequently Asked Questions Does ISO 65 round up or down in US? Size for the warm hand and a clear knuckle; at larger sizes the gap between a ring that slides off and one that jams is only a fraction of a millimetre.
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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.