UK 13.5 is written Z1 because the alphabet runs out; France and Germany keep counting in plain millimetres, labelling the same ring 71.
ISO to UK conversion
ISO millimetre sizes step onto UK letters at roughly one letter per 1.3 mm: ISO 52 is UK L, ISO 54 is UK N, ISO 58 is UK Q. When the circumference passes ISO 70 the UK alphabet is exhausted and the scale continues as Z1 and Z2. Because a letter covers a little over a millimetre, an ISO value on the boundary can read as either of two letters, so the millimetre figure is the one to quote when accuracy matters.
At this size, ISO to UK
One size up from ISO 71, UK steps to UK Z2 while ISO rises 1 to ISO 72. This size runs a 22.6 mm bore.
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 UK? 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.
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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.