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Buying vintage jewelry from Japan or shopping internationally can be difficult when you don't know your exact size. Japan uses a unique numeric sizing system (JIS standard) that does not align with the US/Canada numeric system or the UK/Australia alphabetical system. This instant converter bridges that gap so you can shop with confidence.
Japanese ring sizes typically range from 1 to 27 for adults. Unlike US sizes which increase by quarter or half steps based on fractional inches, the Japanese system is strictly based on the inner circumference and diameter measured in millimeters. For example, a Japanese size 10 has an inner diameter of exactly 16.5mm, which translates to a US size 5.5 and a UK size K½.
If you are converting from a millimeter measurement you took at home, remember that fingers swell throughout the day and fluctuate based on temperature. It is always best to measure your finger in the evening. If your millimeter measurement falls exactly between two Japanese sizes, most jewelers recommend sizing up, especially if you are purchasing a wider band which takes up more space on the finger.