For bands of 8 mm and wider the allowance grows: jewellers move up a quarter to a half size, more if the knuckle is prominent, because a broad band sits flat and grips.
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 M, US moves 0.5 to US 7 while UK steps to UK N. This size runs a 16.9 mm bore. For UK M, the US column here runs US 6, US 6.5, US 7, stepping a half size at a time.
Ring Sizes Around UK M US UK ISO / EU JP / KR Diameter (mm) Circumference (mm) 5 J 49 9 15.7 49.3 5.5 K 51 10 16.1 50.6 6 L 52 12 16.5 51.9 6.5 M 53 13 16.9 53.1 7 N 54 14 17.3 54.4 7.5 O 56 15 17.7 55.7 8 P 57 16 18.1 57
Frequently Asked Questions Does UK M round up or down in US? For a band wider than about 6 mm, round up: jewellers add roughly a half size to keep a broad ring from binding, and that room cannot be added afterwards.
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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.