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101 ring sizes — 52 mm inside is US 6 and Japan 12

Every country calls it something different, but only one thing is being measured — the inside circumference that wraps the finger. From that single number the US number, the Japanese and Korean size, and the EU (ISO 8653) size all follow by rule. All 101 cells from 40.0 mm to 90.0 mm in 0.5 mm steps are worked out here.

One measurement: the inside circumference

Divide the inside circumference by pi and you have the inside diameter; the US number comes from that diameter. The Japanese, Korean and EU sizes come straight from the circumference. Nothing here is copied from a chart — one measurement fixes the other three. It runs backwards too: knowing only US 6 gives a diameter, then 51.9 mm of circumference, and from there the other scales.

The US scale is defined in inches

US 0 has an inside diameter of 0.458 inch (11.63 mm) and each size adds 0.032 inch (0.8128 mm). So diameter = 11.63 + 0.8128 × size, and backwards, size = (diameter − 11.63) ÷ 0.8128. In circumference one size is 2.55 mm, which is where the saying “a size is a tenth of an inch” comes from — that is an approximation; the definition lives on the diameter. Shops only call half sizes, so this table prints the computed value next to the nearest half size.

Japan and Korea: circumference minus 40

Size = inside circumference in mm − 40. Size 13 is 53 mm, which lands near US 6.5; size 9 is 49 mm, near US 5. Korea uses the same numbering. Printed charts usually run from 1 (41 mm) to 27 (67 mm), so the two ends of this table are the same rule extended past what is printed — a shop may not stock those, and millimetres are the safer thing to say.

The EU size is the circumference itself

ISO 8653 names the inside circumference in millimetres and nothing else — 52 mm is size 52. France, Germany and Italy use it, and some jewellers quote it to one decimal. That is also why the addresses here look like iso-52: the number is both the standard size and the millimetres, so it reads the same in every language.

How to measure, and how much it moves

Wrap a strip of paper around the finger, mark where it overlaps, and measure the strip — that length is the inside circumference. If a ring already fits, measure its inside diameter and multiply by pi. Fingers are thinner in the morning, thicker in the evening and swell in heat, so measure twice in a day and take the larger figure. A wide band sits tighter at the same circumference, so allow about half a size, and if the knuckle is the wide part, measure over it too and use whichever is bigger.

The British letter scale is left out

Britain, Ireland and Australia call sizes with letters from A to Z, with a half mark for the steps between. Every other scale on this page follows from one formula; for the letter scale no rule of the same reliability was confirmed. Writing down an approximation would put the half sizes out, and half a size is enough to stop a ring going on. So the column stays empty — telling a British jeweller the millimetres is the exact answer anyway.

At a glance

Inside circumference

Children and little finger40 – 45.5 mm

40 mm
US 1.5JP 0
40.5 mm
US 1.5JP 1
41 mm
US 1.5JP 1
41.5 mm
US 2JP 2
42 mm
US 2JP 2
42.5 mm
US 2.5JP 3
43 mm
US 2.5JP 3
43.5 mm
US 2.5JP 4
44 mm
US 3JP 4
44.5 mm
US 3JP 5
45 mm
US 3.5JP 5
45.5 mm
US 3.5JP 6

The common range46 – 61.5 mm

46 mm
US 3.5JP 6
46.5 mm
US 4JP 7
47 mm
US 4JP 7
47.5 mm
US 4.5JP 8
48 mm
US 4.5JP 8
48.5 mm
US 4.5JP 9
49 mm
US 5JP 9
49.5 mm
US 5JP 10
50 mm
US 5.5JP 10
50.5 mm
US 5.5JP 11
51 mm
US 5.5JP 11
51.5 mm
US 6JP 12
52 mm
US 6JP 12
52.5 mm
US 6.5JP 13
53 mm
US 6.5JP 13
53.5 mm
US 6.5JP 14
54 mm
US 7JP 14
54.5 mm
US 7JP 15
55 mm
US 7JP 15
55.5 mm
US 7.5JP 16
56 mm
US 7.5JP 16
56.5 mm
US 8JP 17
57 mm
US 8JP 17
57.5 mm
US 8JP 18
58 mm
US 8.5JP 18
58.5 mm
US 8.5JP 19
59 mm
US 9JP 19
59.5 mm
US 9JP 20
60 mm
US 9JP 20
60.5 mm
US 9.5JP 21
61 mm
US 9.5JP 21
61.5 mm
US 10JP 22

Larger hands62 – 77.5 mm

62 mm
US 10JP 22
62.5 mm
US 10JP 23
63 mm
US 10.5JP 23
63.5 mm
US 10.5JP 24
64 mm
US 11JP 24
64.5 mm
US 11JP 25
65 mm
US 11JP 25
65.5 mm
US 11.5JP 26
66 mm
US 11.5JP 26
66.5 mm
US 11.5JP 27
67 mm
US 12JP 27
67.5 mm
US 12JP 28
68 mm
US 12.5JP 28
68.5 mm
US 12.5JP 29
69 mm
US 12.5JP 29
69.5 mm
US 13JP 30
70 mm
US 13JP 30
70.5 mm
US 13.5JP 31
71 mm
US 13.5JP 31
71.5 mm
US 13.5JP 32
72 mm
US 14JP 32
72.5 mm
US 14JP 33
73 mm
US 14.5JP 33
73.5 mm
US 14.5JP 34
74 mm
US 14.5JP 34
74.5 mm
US 15JP 35
75 mm
US 15JP 35
75.5 mm
US 15.5JP 36
76 mm
US 15.5JP 36
76.5 mm
US 15.5JP 37
77 mm
US 16JP 37
77.5 mm
US 16JP 38

Thumb and extended sizes78 – 90 mm

78 mm
US 16JP 38
78.5 mm
US 16.5JP 39
79 mm
US 16.5JP 39
79.5 mm
US 17JP 40
80 mm
US 17JP 40
80.5 mm
US 17JP 41
81 mm
US 17.5JP 41
81.5 mm
US 17.5JP 42
82 mm
US 18JP 42
82.5 mm
US 18JP 43
83 mm
US 18JP 43
83.5 mm
US 18.5JP 44
84 mm
US 18.5JP 44
84.5 mm
US 19JP 45
85 mm
US 19JP 45
85.5 mm
US 19JP 46
86 mm
US 19.5JP 46
86.5 mm
US 19.5JP 47
87 mm
US 20JP 47
87.5 mm
US 20JP 48
88 mm
US 20JP 48
88.5 mm
US 20.5JP 49
89 mm
US 20.5JP 49
89.5 mm
US 20.5JP 50
90 mm
US 21JP 50

Worth knowing

  • Inside circumference divided by pi is the inside diameter — every size here comes from that number.
  • US number = (inside diameter in mm − 11.63) ÷ 0.8128.
  • Japan and Korea: size = inside circumference in mm − 40. The EU (ISO 8653) uses the circumference itself.
  • The British letter scale is left out because no rule of the same reliability was confirmed — give the shop the millimetres.

Common questions

Q. How do I measure my ring size?

Wrap a strip of paper around the finger, mark where it overlaps and measure it — that length is the inside circumference. If a ring already fits, measure its inside diameter and multiply by 3.14. Fingers thicken towards evening and swell in heat, so measure twice and take the larger figure.

Q. How many millimetres is US size 6?

An inside diameter of 16.5 mm and an inside circumference of 51.9 mm. That is size 12 in Japan and Korea and 52 in the EU. One full US size is 2.55 mm of circumference, so a half size is about 1.3 mm — roughly the finest difference a paper strip can show.

Q. Why is there no British letter size?

The US, Japanese and EU numbers all follow by formula from one circumference, but for the letter scale no rule of the same reliability was confirmed. An approximation would put the half sizes out, so the column is left empty. British jewellers understand millimetres perfectly well.