Why is inner circumference the canonical unit?

Inner circumference is the international jewelry standard (ISO 8653). This tool converts every input to inner circumference (mm) first, then derives each country's value from there.

How does the Japanese 号 (gō) scale work?

JP 1 号 starts at an inner diameter of 13.0 mm and each whole step adds ≈ 0.318 mm diameter (= 1 mm circumference). Examples: JP 9 ≈ 15.7 mm, JP 17 ≈ 18.3 mm. This tool stores the published odd-number values (1/3/5/...) and linearly interpolates for even numbers.

What's the difference between US and UK sizes?

US sizes step in half sizes (≈ 2.55 mm circumference per full size). UK uses the Wheatsheaf alphabetical scale (≈ 1.30 mm per letter, with A½ / B½ … between letters). Roughly, UK letter position ≈ US × 2 + 1 (e.g. US 6 ≈ UK L–L½).

How do I type a UK size?

Type a single letter (e.g. K) or with a half marker (K1/2 / K½). The numeric position (K = 11, K½ = 11.5) also works. Sizes above Z use Z+1, Z+2 etc.

What does the EU size mean?

EU sizing is simply the inner circumference of the ring in millimetres. For example, EU 50 = 50 mm circumference = ~15.92 mm inner diameter. Used across France, Germany, Italy and most of continental Europe.

Why do brands feel different at the same labelled size?

A wide band feels tighter than a narrow one at the same nominal size — JP 9 in a 1 mm wedding band fits differently from JP 9 in a 6 mm band. Some brands also publish charts shifted by 0.25–0.5 sizes. For something like an engagement ring, always verify with the brand's own diameter (mm) chart.

Is my input uploaded?

No. All computation runs in your browser and the values never leave the page.

Back to Convert

Ring Size Converter (JP 号 ⇄ Inner mm ⇄ US ⇄ UK ⇄ EU)

Convert ring sizes between Japan 号 (1–29), inner diameter (mm), US, UK (Wheatsheaf A–Z+), and EU (inner circumference mm). Handy when sizing a Tiffany / Cartier / BVLGARI / Pandora ring sourced from abroad, or when measuring an existing ring with a caliper. UK accepts letters (K / K1/2 / K½) or the numeric position (11.5). Internally everything is normalised to inner circumference and linearly interpolated from the published odd-number Japanese chart. Brands vary by 0.25–0.5 sizes — always double-check the maker's own diameter (mm) chart for a critical purchase such as an engagement ring. Runs entirely in your browser — no uploads.

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How to use

1) Pick the input unit (Japan 号 / inner diameter mm / US / UK / EU). 2) Enter a size value. UK accepts letters (K / K1/2 / K½) or the numeric position (11.5). 3) The other four units appear in the table instantly. Use Copy on any row to grab a single value. There is no run button; everything is live.

FAQ

Why is inner circumference the canonical unit?: Inner circumference is the international jewelry standard (ISO 8653). This tool converts every input to inner circumference (mm) first, then derives each country's value from there.
How does the Japanese 号 (gō) scale work?: JP 1 号 starts at an inner diameter of 13.0 mm and each whole step adds ≈ 0.318 mm diameter (= 1 mm circumference). Examples: JP 9 ≈ 15.7 mm, JP 17 ≈ 18.3 mm. This tool stores the published odd-number values (1/3/5/...) and linearly interpolates for even numbers.
What's the difference between US and UK sizes?: US sizes step in half sizes (≈ 2.55 mm circumference per full size). UK uses the Wheatsheaf alphabetical scale (≈ 1.30 mm per letter, with A½ / B½ … between letters). Roughly, UK letter position ≈ US × 2 + 1 (e.g. US 6 ≈ UK L–L½).
How do I type a UK size?: Type a single letter (e.g. K) or with a half marker (K1/2 / K½). The numeric position (K = 11, K½ = 11.5) also works. Sizes above Z use Z+1, Z+2 etc.
What does the EU size mean?: EU sizing is simply the inner circumference of the ring in millimetres. For example, EU 50 = 50 mm circumference = ~15.92 mm inner diameter. Used across France, Germany, Italy and most of continental Europe.
Why do brands feel different at the same labelled size?: A wide band feels tighter than a narrow one at the same nominal size — JP 9 in a 1 mm wedding band fits differently from JP 9 in a 6 mm band. Some brands also publish charts shifted by 0.25–0.5 sizes. For something like an engagement ring, always verify with the brand's own diameter (mm) chart.
Is my input uploaded?: No. All computation runs in your browser and the values never leave the page.

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