Ring size converter

Convert a ring size between Japan (号), US, UK letters, EU — or a ruler measurement in mm.

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JP sizes convert by the standard formula (inner circumference ≈ π × (号 + 38) / 3 mm); US, UK, EU and ruler measurements snap to the nearest standard chart row. Real rings differ by maker and band width — confirm in person before buying. JP 1–36, diameter 14–23.5 mm.

How to use

  1. Choose what you know — a size system or a millimetre measurement.
  2. Enter the value.
  3. Read the equivalents in all systems.

What it supports

JP sizes convert by the standard 号 formula; US, UK, EU and ruler inputs snap to the nearest standard chart row and are flagged as approximations. Real rings differ by maker and band width — confirm in person before buying. JP 1–36, inner diameter 14–23.5 mm.

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