Why the UK uses letters and Europe uses millimetres
Ring sizing systems were standardised nationally, before international trade in jewellery mattered, so each market fixed its own scale. Britain chose letters in 1/64-inch steps, America chose numbers, and Europe chose the honest option: the inner circumference in millimetres.
There is no technical reason a British ring size is a letter and an American one is a number. There is only history, and the history is that nobody was coordinating. Each system was fixed by a national trade body at a time when a ring bought in Birmingham was very unlikely to be worn in Boston, and by the time that stopped being true the tools were already made.
What is every sizing system actually describing?
Strip away the labels and every system in the world encodes one of two numbers: the inner diameter of the ring in millimetres, or its inner circumference in millimetres.
Those two are related by π, so they carry identical information. Everything else, the letters and the integers and the decimals, is naming convention layered on top.
This matters practically rather than just historically. It means that any size in any system can be converted to any other exactly, with no approximation, provided you go back to the millimetres rather than looking up one national scale in terms of another. It also means that when a system seems to disagree with a chart, the millimetres are the tiebreaker.
Why does Britain use letters?
The UK scale runs from A upward in half-letter steps, each step representing about a sixty-fourth of an inch of circumference. It was fixed by the British jewellery trade and inherited by Ireland, Australia and New Zealand along with the rest of the imperial toolkit.
Letters have one real advantage: they cannot be mistaken for a measurement. Nobody has ever looked at “N½” and wondered whether it meant millimetres, which is more than can be said for the European system.
They have one real disadvantage, which is that the alphabet runs out. Larger fingers exist past Z, so British sizing continues as Z+1, Z+2, Z+3 and onward. Some retailers write these as Z1 or Z+, and a few jump from Z to “Z½” instead, which is exactly as helpful as it sounds. If you are ordering in that range, send the millimetre diameter alongside the letter.
Why are American sizes numbers?
Because the US scale was built as an arithmetic progression rather than an alphabetical one.
Size 0 sits at a defined diameter and each whole size adds a fixed increment. Written out, the relationship is:
circumference in millimetres = 36.537 + 2.5535 × US size
That makes US sizes convenient to compute with. The difference between a 6 and a 9 is exactly three increments, and you can interpolate a quarter size without consulting anything.
It also makes them completely opaque. The number itself means nothing physical. “Size 7” tells you where a ring sits on an arbitrary scale and nothing about how wide it is, which is why every American jeweller keeps a chart on the wall.
Why do European sizes look so strange?
Because they are not sizes in the American sense at all. A European size is the inner circumference of the ring in millimetres, stated plainly.
A ring marked 54 is 54 mm around the inside. That is a diameter of about 17.2 mm, which is a US 7. The convention is codified as ISO 8653, and Russia uses the same approach.
This is the only system where the number tells you something true without a lookup table, which is why people who work across markets tend to prefer it.
Anyone used to US numbers reads 54 and assumes a mistake, because it is nowhere near a 7. It is not on the same scale. Once you know European sizes are circumferences, the numbers stop looking arbitrary and start looking like the only sensible option in the set.
How do Japanese and Chinese sizes work?
Japan uses its own integer scale running from roughly 1 to 27, fixed by the Japanese jewellery trade on the same principle as the American one: an arbitrary starting point and a fixed step.
Most conversion charts, including the one built into Ring Sizer, treat Chinese sizes as sharing that scale. In practice the two are close enough that the same table serves both, though you will occasionally find sources that disagree by a single step at the extremes of the range.
The trap with both is that they share digits with the US scale while meaning something entirely different. Japanese size 13 is a US 6½, not a US 13. If someone quotes you a bare number in the teens with no country attached, ask before you order.
Convert between all of them
Why has nobody standardised this?
ISO 8653 exists and specifies circumference in millimetres. It has not displaced the national systems, and the reason is economic rather than technical.
Every jeweller owns a set of steel mandrels marked in their own national scale. Every catalogue, every website, every stock system and every trade counter encodes it. Switching means re-marking tools, relabelling inventory and retraining staff, and the cost of all that falls on the trade.
The benefit falls on the customer, and only on the subset of customers who buy across borders. That is a bad trade for the people who would have to pay for it, so the systems persist, and conversion charts persist alongside them.
What should you actually send a jeweller?
Millimetres, and say which kind.
Not a US number, not a UK letter, but the inner diameter in millimetres with the word “diameter” attached to it. That is the one value that survives translation into any market, any decade and any workshop, and any jeweller anywhere can cut metal to it.
Everything else has to be looked up, and looking things up is where the mistakes come from. The full mapping is on the ring size chart, and the US to UK pair, which is the conversion most often needed and most often botched, has its own page at US to UK ring size.