UK 13.5 is written Z1 because the alphabet runs out; France and Germany keep counting in plain millimetres, labelling the same ring 71.
Brand fit: A broad band at US 13.5 grips a large finger firmly, so 71.0mm usually needs a quarter size more than a slim band of equal circumference to clear the knuckle.
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 71, US moves 0.5 to US 14 while ISO rises 1 to ISO 72, so the two scales spread wider. ISO 71 is whole but US 13.5 carries a half, so a rack of full sizes only rounds to the nearest. For ISO 71, the US column here runs US 13, US 13.5, US 14, stepping a half size at a time.
Ring Sizes Around ISO 71 US UK ISO / EU JP / KR Diameter (mm) Circumference (mm) 12 X 67 26 21.4 67.2 12.5 Y 68 27 21.8 68.5 13 Z 70 28 22.2 69.7 13.5 Z1 71 28+ 22.6 71 14 Z2 72 28+ 23 72.3
Frequently Asked Questions Does ISO 71 round up or down in US? Round to clear the knuckle; at the extended sizes a ready-made band is unlikely, so building in the right allowance from the start avoids a second resizing.
ℹ️
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.