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Ring Size Converter

Convert ring sizes across US, UK, EU/ISO, Japan, plus inner circumference and diameter in millimetres. Anchored to circumference — the measurement that doesn't change between countries.

Reviewed by the 247Sizing teamLast updated: 12 August 2026
Enter any value — including a millimetre measurement from a ring that already fits — and every system updates at once. The EU/ISO size is the inner circumference in mm by definition, so it's the most reliable reference. UK half-letter conventions vary slightly between jewellers, so treat the UK letter as the closest match and confirm with a physical sizer.

Convert any ring size

Enter any one value to convert.
⚠️ Measure carefully: fingers swell through the day — size at the end of the day at room temperature. Measuring an existing ring's inner diameter, or our printable ring sizer, beats a guessed size.

How to measure & use it

  1. Measure an existing ring

    Take a ring that fits the right finger and measure its inner diameter across the middle in mm. Enter it under “Diameter”.

  2. Or wrap a strip

    Wrap a thin strip of paper around the finger, mark where it overlaps, and measure the length in mm. That's your circumference — or print our ring sizer.

  3. Read every system

    The converter shows US, UK, EU, JP, circumference and diameter together. EU equals the circumference, so it's the safest cross-check.

  4. Between sizes? Size up

    If you land between two, choose the larger — a slightly loose ring can be resized down more easily than a tight one can be stretched.

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How these conversions are calculated

Ring sizes are anchored to inner circumference in millimetres. The EU/ISO size equals that circumference (ISO 8653); US sizes follow the standard inner-diameter formula (0.032in per size); UK letters and Japanese numbers follow cross-checked international charts. UK half-letter conventions vary slightly between jewellers, so treat UK letters as the closest equivalent and confirm with a physical sizer.

Sources: ISO 8653:2016 — EU ring size = inner circumference (mm); US/Canada standard — inner diameter, 0.032in per size from 0.458in base; Cross-checked international ring charts (ringchart.co.uk, antoanetta.com, miniwebtool, 2026).