The Japanese sizing standard is used through much of South-East Asia, as well as in some South American countries. However, there are other sizing systems that you may encounter, particularly in Taiwan, Korea, and Hong Kong. These sizing systems use numeric scales that are close to one another. As an example a US size 11, is slightly smaller than Japanese size 24. However, in Hong Kong it is reported to be 24 3/4, Korea a 23, and in Taiwan a size 19 based on local sizing scales.
For this reason it would be best to try to get as much information as possible on your finger size. If you are measuring your finger in Asia, ask the person measuring to provide you with not only the size but also the diameter measurement of the inside of the ring that fits you well (see instructions at this page). This way you can match it more reliably to the US finger size. If you need to convert your US finger size to an Asian size, convert your size not only to the Japanese size, but also to multiple systems; the internal diameter being the most important.
The Japanese is a linear sizing scale that assigns values to 1/3 millimeter increments of internal diameter with size 1 being 13mm in diameter. As it uses diameter to establish sizes, it does have some correlation with the German ring size system.
US Size
Japan Size
Diameter mm
1 3/8
1
13
1 3/4
13.1
2
13.3
2 1/4
13.5
2 1/2
3
13.7
2 3/4
13.9
2 7/8
4
14
14.1
3 1/4
5
14.3
3 1/2
14.5
3 3/4
6
14.7
14.9
4 1/8
7
15
4 1/4
15.1
4 1/2
8
15.3
4 3/4
15.5
9
15.7
5 1/4
15.9
5 3/8
10
16
5 1/2
16.1
5 3/4
11
16.3
16.5
6 1/4
12
16.7
6 1/2
16.9
6 5/8
17
6 3/4
17.1
17.3
7 1/4
17.5
7 1/2
17.7
7 3/4
17.9
7 7/8
18
18.1
8 1/4
18.3
8 1/2
18.5
8 3/4
18.7
18.9
9 1/8
19
9 1/4
19.2
9 3/8
20
19.3
9 1/2
19.4
9 3/4
19.6
9 7/8
21
19.7
19.8
10 1/4
22
10 1/2
20.2
10 5/8
23
20.3
10 3/4
20.4
20.6
11 1/8
24
20.7
11 1/4
20.8
11 1/2
25
11 3/4
21.2
11 7/8
26
21.3
21.4
12 1/4
21.6
12 3/8
27
21.7
12 1/2
21.8
12 3/4
28
22.2
13 1/8
29
22.3
13 1/4
22.4
13 1/2
22.6
13 3/8
30
22.7
13 3/4
22.8
31
14 1/4
23.2
14 3/8
32
23.3
14 1/2
23.4
14 3/4
23.6
14 7/8
33
23.7
23.8
15 1/4
34
15 1/2
24.2
15 5/8
35
24.3
15 3/4
24.4
24.6
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