Ring size·79.5 mm
Inside circumference
79.5 mm
US 17 · JP 40 · EU 79.5
Inside diameter 25.31 mm

79.5 mm inside — US 17, Japan 40

An inside circumference of 79.5 mm is an inside diameter of 25.31 mm. The US formula gives 16.83, so the size called in shops is 17; Japan and Korea say 40, and the EU (ISO 8653) size is 79.5.

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 Canada17
  • Japan and Korea40
  • EU · ISO 865379.5
Inside circumference
79.5 mm
Inside diameter
25.31 mm
Diameter in inches
0.996 in
Nearest US half size
US 17 · 79.95 mm
Gap to that size
-0.45 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

79 mm
US 16.5JP 39
80 mm
US 17JP 40

Nearby sizes

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

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 79.5 mm inside?

The formula gives 16.83, and the half size a shop would call is 17. US 17 measures 79.95 mm, so this cell sits -0.45 mm from it.

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

Size 40, and 79.5 in the EU (ISO 8653). The Japanese and Korean number is the circumference minus 40, so size 40 is 80 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”. 79.5 mm of circumference is 25.31 mm (0.996 in) across, and adult ring fingers mostly land between 46 and 62 mm. Above 78 mm you are in thumb and extended sizes.