Ring Size Conversion Chart — KR, US, UK, EU Circumference and Inner Diameter

Inner ring circumferences from 40 to 73 mm in 1 mm steps, with Korean ring sizes (ho), Japanese 号, US and Canadian, UK and Australian letter, and European (ISO 8653) sizes plus inner diameter.

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Ring sizes are based on inner circumference

The international standard ISO 8653 defines ring size directly as inner circumference (mm). A ring with a 52 mm circumference is size 52. Most of Europe uses this number. All other systems are calculated from this circumference or from the inner diameter.

US and Canada: inner diameter (mm) = 0.8128 × size + 11.63. One size step is about 0.81 mm (0.032 inch) in inner diameter and about 2.55 mm in circumference.
UK, Ireland, Australia: letter sizes (BS 6820:1987). C is a 40 mm circumference, and each letter adds 1.25 mm. After Z come Z+1, Z+2 and so on.
Japan: an inner diameter of 13 mm is 1号, and each additional 1/3 mm of inner diameter adds 1号 (13号 = circumference of about 53.4 mm).
Korean ring sizes (ho): the Korean jewelry industry generally uses the same numbering as Japanese 号. However, no single official standard could be confirmed, so sizes can differ by about one number from shop to shop.

Ring size conversion chart

Inner circumference	Inner diameter	KR ring size (ho)	JP 号	US·CA	UK·AU
40 mm	12.7 mm	1	1号	1¼	C
41 mm	13.1 mm	1	1号	1¾	D
42 mm	13.4 mm	2	2号	2¼	D½
43 mm	13.7 mm	3	3号	2.5	E½
44 mm	14.0 mm	4	4号	3	F
45 mm	14.3 mm	5	5号	3¼	G
46 mm	14.6 mm	6	6号	3¾	H
47 mm	15.0 mm	7	7号	4	H½
48 mm	15.3 mm	8	8号	4.5	I½
49 mm	15.6 mm	9	9号	5	J
50 mm	15.9 mm	10	10号	5¼	K
51 mm	16.2 mm	11	11号	5¾	L
52 mm	16.6 mm	12	12号	6	L½
53 mm	16.9 mm	13	13号	6.5	M½
54 mm	17.2 mm	14	14号	6¾	N
55 mm	17.5 mm	15	15号	7¼	O
56 mm	17.8 mm	15	15号	7.5	P
57 mm	18.1 mm	16	16号	8	P½
58 mm	18.5 mm	17	17号	8.5	Q½
59 mm	18.8 mm	18	18号	8¾	R
60 mm	19.1 mm	19	19号	9¼	S
61 mm	19.4 mm	20	20号	9.5	T
62 mm	19.7 mm	21	21号	10	T½
63 mm	20.1 mm	22	22号	10¼	U½
64 mm	20.4 mm	23	23号	10¾	V
65 mm	20.7 mm	24	24号	11¼	W
66 mm	21.0 mm	25	25号	11.5	X
67 mm	21.3 mm	26	26号	12	X½
68 mm	21.6 mm	27	27号	12¼	Y½
69 mm	22.0 mm	28	28号	12¾	Z
70 mm	22.3 mm	29	29号	13	Z+1
71 mm	22.6 mm	30	30号	13.5	Z+2
72 mm	22.9 mm	30	30号	14	Z+2½
73 mm	23.2 mm	—	—	—	Z+3

Notes on using the chart

US sizes are shown in 0.25 steps. Shops usually sell them in 0.5 steps.
A ½ in UK letter sizes is a half size (0.625 mm of circumference).
Wide bands of 6 mm or more cover more of the finger and feel tighter, so people usually choose half to one size larger.
Fingers change thickness during the day and across seasons. A measurement taken on a warm afternoon is more reliable than one on a cold morning.

Measuring methods — paper strip, an existing ring, and comparing with the on-screen circle — are explained in How to measure your ring size.

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