Ring Size Converter

This page converts a size you already have, live across every field. If you are measuring a finger and choosing a band, the ring size and band width allowance page adds the two things this one deliberately leaves out: how much a wide band tightens the fit, and why a comfort-fit profile wears half a size larger. Both pages use the same scale definitions, so the numbers agree.

Ring sizes are based on the inner circumference of the ring.

US/Canada sizes use a numbered scale (typically 3–16). UK/Australia uses letters (A–Z+), stepping in half letters. EU/ISO uses the inner circumference in millimeters. Japan uses a numbered scale (1–30).

The three lettered and numbered scales all step by different amounts, which is the whole reason a chart is needed. A full US size is 2.55 mm of circumference. A full UK letter is 1.275 mm, so two UK letters make roughly one US size. A Japanese size is 1.05 mm, so about two and a half Japanese sizes make one US size. Nothing lines up, and a number that looks familiar in one system means something quite different in another.

Conversion formulas (using inner circumference in mm):

• US size = (circumference_mm − 36.5) / 2.55
• EU size = circumference_mm (rounded to nearest 0.25)
• Japan size = (diameter_mm − 13.0) × 3 + 1
• Diameter (mm) = circumference_mm / π

The Japanese scale is the one worth explaining, because it is defined on diameter rather than circumference. Under the Japanese Industrial Standard, size 1 has a 13.0 mm inner diameter and every step up adds exactly one third of a millimetre. Work it the other way and diameter = 13.0 + (size − 1) / 3. That is why a US 7 is a Japanese 14 and not something in the twenties: the Japanese step is roughly a third the size of a US one, but it starts from a much smaller ring.

Common reference sizes:

Note: Sizes are approximate. Manufacturing tolerances and different standards may cause slight variations.

The reason EU sizing feels the most logical is that it’s just the inside circumference in millimeters: a size 54 ring measures 54 mm around. US and Japan use abstract numbers and the UK uses letters, all mapping back to the same physical diameter, which is why a conversion chart is the only sane way to shop across systems. The geometry underneath is simple, circumference equals diameter times pi, so a 17.3 mm diameter works out to about 54 mm around, a US 7.

A few fitting realities the chart can’t capture. Size for your knuckle, not the base of the finger, if the knuckle is the wider part, or the ring won’t go on; just not so loose that it spins freely, which is the usual overcorrection. Band width matters too: a wide band feels tighter than a thin one at the same nominal size, so for anything over about 6 mm wide, order a quarter to half size up. And most rings can only be resized one or two sizes, while full-eternity bands and tension settings often can’t be resized at all, so getting it right the first time is worth the trouble.

Measuring a finger without a sizer. Cut a strip of paper about 6 mm wide and 100 mm long, or use a length of non-stretchy string. Wrap it around the base of the finger, snug but not tight, and mark where it overlaps. Lay it flat against a ruler and read the millimetres from the end to the mark. That figure is the inner circumference, which goes straight into the EU field above and gives you every other system at once. Three points make this measurement worth redoing rather than trusting the first try:

• Measure at the end of the day, at room temperature. Fingers are noticeably smaller first thing in the morning and in the cold, and larger after exercise, salt, or heat. A morning measurement in winter can be a full size under the truth.
• Check the knuckle too. If the knuckle measures more than about 2 mm larger than the base, take the average of the two rather than either one, or the ring will either not pass the knuckle or spin once it is on.
• Do not use a stretchy thread. Anything with give reads small, which is the single most common way a home measurement comes out a size or two under.