UK 13.5 is written Z1 because the alphabet runs out; France and Germany keep counting in plain millimetres, labelling the same ring 71.
UK to US conversion
Reading UK letters into US numbers reverses that alphabet: UK L is US 6, UK N is US 7, UK Q is US 8.5, one letter for every half a US size. Past UK Z the scale runs out of letters and switches to Z1 and Z2, which answer US 13.5 and 14. Since one letter can land on a US whole or half, a UK buyer converting to US should confirm the half, especially before ordering a size that cannot be adjusted after casting.
At this size, UK to US
One size up from UK Z1, US moves 0.5 to US 14 while UK steps to UK Z2. This size runs a 22.6 mm bore. For UK Z1, the US column here runs US 13, US 13.5, US 14, stepping a half size at a time.
Ring Sizes Around UK Z1 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 UK Z1 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.
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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.