AU to Japan Ring Size Conversion

Australian sizes are letters

AU ring sizes progress through letters, for example K, L, M, N, O and P. Half-letter steps may be available. Australian and UK letter labels are normally treated as the same sizing system.

Japanese sizes are JCS numbers

Japanese jewellery commonly uses JCS (Japan Custom Size) numbers such as 9号, 10号, 14号 and 20号. Japanese manufacturers publish each number with an inner diameter and circumference.

Use the nearest physical match

AU N½ is close to 54.4 mm circumference and therefore converts to approximately Japan 14. Different charts can place an AU half size between two Japanese whole numbers, so the AU to Japan Ring Size Conversion calculator preserves the millimetre values.

Do not convert by simple arithmetic

There is no dependable rule such as subtracting a fixed number from an AU letter. Use a chart or physical measurement because both scales have their own increments.

Why Australian and Japanese labels look unrelated

Australia inherited the British alphabetical sizing system, while Japan developed the JCS numbered system. The sequences were not designed as simple translations of each other, so matching must occur through physical dimensions.

How JCS sizes progress

Japanese JCS sizes generally increase by approximately one millimetre of circumference per whole number. Japanese size 10 is near 50.3 mm circumference, size 14 near 54.5 mm and size 20 near 60.7 mm.

Why two AU sizes can map to one Japanese size

Australian charts include half letters, while many Japanese charts show whole JCS numbers. Both AU N and N½ can therefore be listed near Japan 14 depending on rounding.

Why diameter is useful for an existing ring

An existing ring can be measured with digital callipers. Measuring the internal opening to one decimal place often produces a more repeatable result than wrapping string around a finger.

Why circumference is useful for a finger

A paper strip or non-stretch tape measures the distance around the finger directly. That value can be matched to both systems.

Understanding Japanese listing language

リングサイズ means ring size. 内径 means inner diameter, 内周 means inner circumference and 号 marks the Japanese size number.

Why final confirmation matters

International conversion tables are practical references, not guarantees of identical fit. Band width, internal profile, manufacturing tolerance and available JCS increments can affect the final choice.