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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.

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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Frequently asked questions

How do I measure my ring size at home?

Measure the inner diameter of a ring that already fits across the middle in millimetres, or wrap a strip of paper around your finger and measure the length (that's the circumference). You can also print our ring sizer.

What is the EU/ISO ring size?

The EU/ISO ring size is simply the inner circumference in millimetres — an EU 54 measures 54 mm around the inside. That makes it the most reliable cross-reference between countries.

What is a US 7 in UK ring size?

A US 7 is approximately a UK O, with an inner circumference around 54 mm. UK half-letter conventions vary slightly between jewellers, so confirm with a physical sizer.

Should I size up or down if I'm between ring sizes?

Size up. A slightly loose ring can be resized down or fitted with an insert, whereas a tight ring is harder and riskier to stretch. Also measure at the end of the day when fingers are largest.

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).