Ring Inner Diameter (mm) to US / Canada Size

Every ring size in the world describes one physical thing: the inner circumference of the band, and the inner diameter that follows from it (circumference divided by π). Each country just writes that number a different way. The EU / ISO 8653 standard is the most honest about it — the size is the circumference in millimetres, so an EU 54 ring measures 54 mm around the inside. The France, Spain, Italy and Switzerland family (and Brazil) uses the same circumference minus forty, so that EU 54 is a French 14. The US and Canada scale runs on the diameter instead: each whole size adds about 0.81 mm of diameter (roughly 2.55 mm of circumference), which is why US half and quarter sizes exist. The UK, Ireland and Australia share a letter ladder from A upward, with a half-size between each letter, while Japan, China and Korea share one numeric scale and India uses another. Because letter and numeric ladders are fixed national standards rather than formulas, a size in one system can land exactly between two sizes in another — and different jewellers' charts sometimes disagree by half a size on the letter conversions, so treat a converted letter as ±½. The one measurement that never lies is the millimetre value: a 17.3 mm inner diameter (54.4 mm circumference) is a US 7, a UK N½, an EU 54, a Japanese 14 and an Indian 15 no matter what is engraved inside the band. To size yourself at home, wrap a strip of paper around the base of the finger, mark where it overlaps, and measure that length in millimetres — that is your circumference, and everything else converts from it. Measure at the end of the day when fingers are largest, avoid sizing over a knuckle that is much wider than the base, and remember that wide or heavily set bands wear tighter, so many jewellers suggest sizing up a quarter to a half for those.