指輪サイズの国際標準規格：ISO 8653:2016と日本の号数の関係

Unlike shoe or clothing sizes which vary wildly across apparel manufacturers, jewelry sizing is governed by precise mathematical formulas and international metrological standards.

The worldwide benchmark for ring sizing is ISO 8653:2016 (Jewellery — Ring-sizes — Definition, measurement and designation). Understanding how this standard maps to regional conventions ensures you never buy the wrong size when ordering jewelry from international brands.

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The 4 Primary Global Sizing Systems

1. The Metric Circumference System (ISO 8653 / Europe)

Standardized across France, Germany, Austria, and Scandinavia, the ISO ring size is simply the inner circumference of the ring in millimeters.

• An ISO Size 54 has an inside circumference of exactly 54.0 mm (diameter = 17.19 mm).

2. The US and Canadian Numerical System

The American standard is based on internal diameter measured in inches:

• Formula: US Size = (Inside Diameter in inches - 0.458) × 64 + 1
• Each full size increases the inside diameter by exactly 0.032 inches (0.8128 mm).

3. The British and Australian Alphabetical System

The UK scale uses letters from A to Z, plus half sizes (e.g., M, M 1/2, N):

• Size A starts at an inside circumference of 37.5 mm.
• Each consecutive half-size increases circumference by 0.625 mm.

4. The Japanese and East Asian Scale

Used extensively in Japan, China, Korea, and Hong Kong:

• Runs numerically from Size 1 (13.0 mm diameter) upward in 1/3 mm diameter steps.

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