US 10, in the men's range, is about 62 mm around and 19.8 mm across, which Japan and Korea number as their 21 on the shared 号 and 호 scale.
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 T, US moves 0.5 to US 10.5 while UK steps to UK U. This size runs a 19.8 mm bore. For UK T, the US column here runs US 9.5, US 10, US 10.5, stepping a half size at a time.
Ring Sizes Around UK T US UK ISO / EU JP / KR Diameter (mm) Circumference (mm) 8.5 Q 58 18 18.6 58.3 9 R 60 19 18.9 59.5 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
Frequently Asked Questions Does UK T round up or down in US? Clear the knuckle first: on a broad-knuckled hand a 10 that passes the joint may spin at the base, which sizing beads can fix, whereas a stuck ring cannot.
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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.