Ring size·89.5 mm
Inside circumference
89.5 mm
US 20.5 · JP 50 · EU 89.5
Inside diameter 28.49 mm

89.5 mm inside — US 20.5, Japan 50

An inside circumference of 89.5 mm is an inside diameter of 28.49 mm. The US formula gives 20.74, so the size called in shops is 20.5; Japan and Korea say 50, and the EU (ISO 8653) size is 89.5.

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 Canada20.5
  • Japan and Korea50
  • EU · ISO 865389.5
Inside circumference
89.5 mm
Inside diameter
28.49 mm
Diameter in inches
1.122 in
Nearest US half size
US 20.5 · 88.88 mm
Gap to that size
+0.62 mm
Where it sits
Thumb and extended sizes

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

89 mm
US 20.5JP 49
90 mm
US 21JP 50

Nearby sizes

90 mm
US 21JP 50
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

Thumb and extended sizes

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. What US size is 89.5 mm inside?

The formula gives 20.74, and the half size a shop would call is 20.5. US 20.5 measures 88.88 mm, so this cell sits +0.62 mm from it.

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

Size 50, and 89.5 in the EU (ISO 8653). The Japanese and Korean number is the circumference minus 40, so size 50 is 90 mm, while the EU size is the circumference itself.

Q. Which finger is this size for?

It sits in the band called “Thumb and extended sizes”. 89.5 mm of circumference is 28.49 mm (1.122 in) across, and adult ring fingers mostly land between 46 and 62 mm. Above 78 mm you are in thumb and extended sizes.