Ring size·63 mm
Inside circumference
63 mm
US 10.5 · JP 23 · EU 63
Inside diameter 20.05 mm

63 mm inside — US 10.5, Japan 23

An inside circumference of 63 mm is an inside diameter of 20.05 mm. The US formula gives 10.36, so the size called in shops is 10.5; Japan and Korea say 23, and the EU (ISO 8653) size is 63.

At a glance

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.

  • US and Canada10.5
  • Japan and Korea23
  • EU · ISO 865363
Inside circumference
63 mm
Inside diameter
20.05 mm
Diameter in inches
0.79 in
Nearest US half size
US 10.5 · 63.35 mm
Gap to that size
-0.35 mm
Where it sits
Larger hands

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.

One step either way

0.5 mm smaller · 0.5 mm larger

62.5 mm
US 10JP 23
63.5 mm
US 10.5JP 24

Nearby sizes

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

Larger hands

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

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. What US size is 63 mm inside?

The formula gives 10.36, and the half size a shop would call is 10.5. US 10.5 measures 63.35 mm, so this cell sits -0.35 mm from it.

Q. What are the Japanese, Korean and EU sizes?

Size 23, and 63 in the EU (ISO 8653). The Japanese and Korean number is the circumference minus 40, so size 23 is 63 mm, while the EU size is the circumference itself.

Q. Which finger is this size for?

It sits in the band called “Larger hands”. 63 mm of circumference is 20.05 mm (0.79 in) across, and adult ring fingers mostly land between 46 and 62 mm. Above 78 mm you are in thumb and extended sizes.