US to UK ring size conversion

US numbers and UK letters describe exactly the same rings, but the two scales are offset from each other and step at different intervals. That combination makes this the conversion people most often get wrong, and the error is usually invisible until the ring arrives.

What is the US to UK conversion chart?

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Why do US whole sizes land on UK half letters?

This is the single most common mistake in the conversion: reading “US 6 equals UK L” when it is actually L½.

The arithmetic explains it. One full US size is about 2.55 mm of circumference. One full UK letter is about 1.27 mm, which is almost exactly half. So one US size spans two UK letters, and most US whole sizes land on a UK half letter rather than a whole one.

Drop the half and you are half a US size out. Do it twice in a round-trip conversion, US to UK and back again, and you can end up a full size from where you started without ever having made what looks like an error.

Why do the two scales differ at all?

They were standardised separately, for the same trade, in the same century, and nobody reconciled them afterwards.

The UK scale was fixed by the British jewellery trade using letters at intervals of a sixty-fourth of an inch of circumference, starting from a nominal A. It was inherited by Ireland, Australia and New Zealand along with the rest of the imperial toolkit.

The US scale was built as a numeric progression, with size 0 at a defined diameter and each size adding a fixed increment, which is why it can be expressed as a formula rather than a table.

Both are arbitrary labels on the same physical measurement, neither is more correct than the other, and neither is going away. Every jeweller in both markets owns mandrels marked in their own scale, and the cost of switching falls on the trade while the benefit falls on the minority of customers who buy across borders.

What happens above Z?

The UK alphabet runs out before large fingers do, which produces one of the less elegant corners of jewellery notation.

Above Z, British sizing continues as Z+1, Z+2, Z+3 and onward, each still a half-letter step. A US 13 is a UK Z+1.

Retailers write these inconsistently. Some use Z1 or Z2, some use Z+ on its own, and a few jump straight from Z to “Z½”, which means something different again. If you are ordering anywhere in this range, send the millimetre diameter alongside whatever letter you have been given, because it removes all doubt at no cost.

Does this chart apply to Ireland and Australia?

Yes. Ireland, Australia and New Zealand all use the UK letter scale, so a UK N½ is an Australian N½ and an Irish N½.

If a retailer in any of those markets quotes you a letter, read it off the UK column of the chart above. There is no separate Australian scale to convert to, despite the occasional website implying otherwise.

South Africa also uses the UK scale, as do several other Commonwealth markets, which makes the letter system considerably more widespread than its name suggests.

Canada is the exception worth remembering. Despite the Commonwealth connection it uses the US numeric scale, so a Canadian retailer quoting a “size 7” means the US 7, not a UK letter.

Which conversions cause the most trouble in practice?

Three, and knowing them in advance is most of the defence.

US whole sizes to UK letters, because of the half-letter rule above. If a chart gives you a bare letter with no half, check it against a second source before ordering.

UK letters above V, where retailers write the same size in three or four different ways and the notation stops being self-explanatory.

Anything a seller has already converted. If an independent maker quotes you a UK letter for a ring they measure in millimetres, you are inheriting their conversion as well as making your own. Ask what inner diameter they actually work to and convert once, yourself, from that.

How do you check a conversion is right?

Use the millimetres as the arbiter, because they are the only value both scales agree on.

Look up your US size and your UK letter separately and compare the diameter each one gives. If the two land on the same figure, the conversion is correct. If they differ by more than about 0.2 mm, one of them is wrong and it is almost always the letter.

This takes about fifteen seconds with the chart above and it catches the half-letter error reliably, which makes it worth doing on any purchase expensive enough to care about.

Which value should you give a jeweller?

Match the system to the market. A British, Irish or Australian jeweller wants the letter. An American or Canadian one wants the number.

If you are not sure, or you are ordering from anywhere else in the world, send the inner diameter in millimetres and say that it is the diameter. It is the only value that means the same thing everywhere, it needs no chart, and every jeweller can cut metal to it directly.

This matters most with independent makers and vintage dealers, who frequently quote a size they have converted themselves from a measurement. Asking for the original measurement removes their conversion from the chain, and each conversion you remove is one fewer place for a half size to go missing.

The full international table