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Ring Size in mm to US Size

Convert ring sizes between the US, UK, EU/ISO and Japanese systems, or start from a measured diameter or circumference in millimetres.

How to measure at home

No ring at hand: wrap a thin strip of paper or a thread snugly around the base of the finger, mark where it overlaps and measure the length in mm — that is the circumference. Fingers swell during the day, so measure in the evening at room temperature.

With a ring that fits: measure the inner diameter of the ring in mm from edge to edge across the centre, then convert from “Inner diameter, mm”. Pick a ring worn on the same finger of the same hand.

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Reading the size
Matching the ISO 8653 table
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Result

This page opens the ring size converter preset to start from a millimetre measurement: the inner diameter of a ring that already fits. Measure straight across the widest inside part of the band, enter the number — say 17.3 — and the converter returns the US size plus the full standard row in every other system.

Diameter is the easiest thing to measure accurately at home: lay the ring on a ruler or use callipers, and read edge to edge through the centre, excluding the metal itself. A measurement of 16.5 mm lands near US 6, 17.3 mm near US 7 and 18.2 mm near US 8; values in between are interpolated, not rounded away.

If you have no ring to measure, switch the source system to inner circumference and wrap a strip of paper around the finger instead — the measuring instructions are right below the form. For wide bands, add about a quarter size to the result, since they wear tighter than thin ones.

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Accepted Formats & Rules

  • ISO 8653 based conversion table with 44 standard sizes.
  • Accepts UK letter sizes including halves, like M or M½.
  • Millimetre inputs are interpolated between table rows for an exact match.

Key Benefits

✓ Convert between five sizing systems and raw millimetre measurements in one place.
✓ See the full matching row at once — US, UK, EU/ISO, Japan, diameter and circumference.
✓ Includes simple instructions for measuring your finger or an existing ring at home.

Use Cases

  • • Convert a ring size from an American online store to the EU size your local jeweller uses.
  • • Find your own size by measuring an existing ring's inner diameter or your finger's circumference.
  • • Check a UK letter size like M½ against the US, European and Japanese scales before ordering a gift.