Ring Size Converter
Convert between US, UK, EU, and JP ring sizes, or work from a finger measurement in mm or inches.
Ring size converter
Updates as you typeHow to measure a ring (3 ways)
- Measure an existing ring. Pick one that fits the finger you're sizing. Measure the inside diameter with a ruler or caliper — switch to Diameter above and enter the value.
- String or floss method. Wrap it snugly around the base of the finger, mark where it overlaps, lay it flat, and measure in millimetres. That's your circumference.
- Printable sizer. Many jewellers offer a paper strip you wrap around the finger. The marked number is usually the EU size (≈ inner circumference in mm).
Fingers swell in heat and shrink in cold. Measure at the end of a warm day, and if you're between sizes, size up — a ring that can't pass the knuckle is a ring you can't wear.
Full ring size chart
Your current size is highlighted| US | UK | EU | JP | Diameter | Circumference |
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How ring-size conversion works
Ring sizes come from the inner circumference of the ring — the distance around the finger. The four major systems use different starting points and different step sizes for the same physical measurement:
- EU
- Inner circumference in millimetres, rounded. EU 54 ≈ 54 mm around. The most intuitive system to explain.
- JP
- An integer index tied to circumference but not linearly — skips numbers irregularly (JP 4 → 5 → 7, no 6).
- US
- Whole and half numbers. Each half-size adds roughly 0.4 mm of diameter (~1.3 mm of circumference).
- UK / Ireland / Australia
- Letter-based. Half-sizes appear as "N½", "O½" etc. The same alphabet is used by several Commonwealth countries.
Given a circumference C in mm, the inner diameter is D = C / π. Most online converters match against a lookup table because the mapping between regions isn't a clean formula — jewellers rounded differently when each system was standardised, and those historical rounding choices are what the table preserves.
Common ring-sizing questions
My finger is a different size in the morning vs. the evening — which do I measure?
Measure at the end of a warm day, when your fingers are at their largest. A ring that fits in the morning may not pass the knuckle by 3 pm in summer. If you live somewhere with cold winters, take the measurement at room temperature, not straight from outside.
Should I size up for a wide band?
Yes — rings wider than about 6 mm feel tighter at the same nominal size because more of the band sits against your skin. Go half a size up for bands 6–8 mm wide, and a full size up for bands over 10 mm.
My knuckle is wider than the base of my finger. What size do I order?
Size so the ring just clears the knuckle, then accept a slightly loose fit at the base. A good jeweller can add small metal "beads" (sizing balls) inside the band to hold the ring upright without changing the outer size.
Are half-sizes available everywhere?
US and Japan routinely stock half-sizes. EU numbers are in whole millimetres, so the nearest half-size lands on the next whole EU up. The UK grid has some half-letters (J½, N½, etc.) but many jewellers only stock the whole letters — if your measurement falls between two UK sizes, size up.
Can I trust a paper ring sizer I printed at home?
Only after calibrating. Printers scale differently — hold the printed sheet against a credit card (85.60 mm wide) and check the reference bar matches. Use the "Calibrate to your screen" slider in the result panel for a digital check before printing.
My measurement is between two sizes — which do I pick?
Size up. A ring that's slightly loose can be resized down cheaply; a ring that won't go over the knuckle usually needs a full re-shank, which is expensive and removes any hallmark. The tool shows a "between sizes" hint when your measurement falls near the midpoint.