Ring Size Calculator

Frequently Asked Questions

How is ring size measured?

Inner diameter in millimeters is the universal measurement; circumference = π × diameter. A 17.3 mm inner diameter is roughly 54.4 mm circumference, which is US 7, UK N½, EU 54, JP 14.

How do US, UK, EU and JP sizing systems differ?

US/Canada use a numeric scale (3–13+ in half sizes), UK/Australia use letters (A–Z), EU sizes equal the inner circumference in mm, and Japan uses a 1–27 scale roughly equal to the EU number minus 40. A US 6 = UK L½ = EU 51.9 = JP 12.

How do I measure my finger at home?

Wrap a strip of paper or string around the base of the finger, mark where it overlaps, and measure that length in mm - that's the circumference. Measure at the end of the day when fingers are largest, and avoid measuring when cold.

Should I size up for wide bands?

Yes. Bands wider than ~6 mm fit tighter than thin ones, so size up by a quarter to a half US size. Comfort-fit (domed inner) rings also generally fit a quarter size larger than standard flat-inside bands.

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How This Calculator Works

Ring sizes are quoted in at least four incompatible systems - US/Canada uses numbered half steps, the UK and Ireland use letters of the alphabet, continental Europe quotes the inner circumference in millimetres, and Japan uses its own numeric series - so a ring that fits perfectly in one chart can be a frustrating two sizes off in another. The single physical reality behind every system is the inner diameter of the band (in millimetres). This calculator converts between all four sizing conventions by translating whatever you provide - a US number, a measured inner diameter, or a measured circumference - into the underlying diameter, and then re-expressing it in every other system. It is the safest way to translate a vintage UK letter to a modern US size, to interpret an EU listing that quotes only millimetres, or to size a gift ring from a strip of paper wrapped around the recipient's finger and measured with a ruler.

Formula / Method

The circle relationship C = π × d ties diameter to circumference, so any measurement of one yields the other. US sizing in this tool uses the common jeweller mapping where US 3 corresponds to a 14.1 mm inner diameter and each whole US size adds about 0.8 mm of diameter (so each half size adds 0.4 mm). UK letters step in roughly 1.25 mm increments of circumference starting near 37.5 mm at A. EU size is simply the inner circumference rounded to the nearest millimetre - an EU 54 ring measures 54 mm around the inside. Japanese sizes start near JP 1 = 13.0 mm diameter and add about 0.3 mm per whole size. All conversions are reversible, and the input modes let you start from whichever measurement you actually have.

Worked Example

A US 7 ring has inner diameter 14.1 + (7 − 3) × 0.8 = 17.3 mm, and circumference 17.3 × π ≈ 54.4 mm. That is EU 54, roughly UK N, and JP 15. Going the other way, if you wrap a paper strip around the finger and measure 56.0 mm, that is 56.0 ÷ π ≈ 17.83 mm diameter - about US 7¾, UK P, EU 56, JP 16. Notice the half-size resolution: a difference of just 0.4 mm in diameter is one US half size, which is why precise measurement matters far more than the system you choose.

Common Mistakes

Measuring a knuckle instead of the base of the finger - the ring must pass over the knuckle but settle at the base, so size for the larger of the two.

Measuring a cold hand - fingers swell up to half a size in heat and shrink in cold; aim for a normal-temperature midday reading.

Wrapping a thick string and measuring its outer rather than inner circumference, inflating the result by twice the string thickness.

Confusing inner diameter with the band's outer diameter or with the diameter of a stone setting.

Sizing wide bands (≥ 6 mm) the same as thin ones - wide bands feel tighter and usually need a half-size up.

Tips & FAQ

What is the most reliable way to measure? Use a plastic or paper ring sizer that you slip on like a belt, or wrap a thin strip of paper around the base of the finger, mark the overlap, and measure the marked length in millimetres - that is the inner circumference, which divided by π gives diameter. Half sizes? US and JP have half steps; UK uses ½ letters (e.g. K½) between letters; EU advances one millimetre at a time. Sister-size for resizing? Most plain bands can be safely resized one to two US sizes either way without distorting the shank or any stones. Wide-band rule: for bands wider than about 6 mm, order a half size larger than your normal narrow-band size, because the extra surface area grips more of the finger. Surprise gift? Borrow a ring she already wears on the same finger and measure its inner diameter with a digital calliper - far more accurate than guessing from a tracing.

Frequently Asked Questions

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