US to UK Ring Size Conversion Explained
How American numeric ring sizes map to British letter sizes, why the two systems drift apart, and how to convert without guessing.
5 min read
The United States sizes rings with numbers and half sizes; the United Kingdom uses letters of the alphabet, sometimes with a half step. The two systems describe the same physical circumference, so conversion is always possible — but there is no single arithmetic rule, which is why a chart or a converter is the safe way to do it.
The relationship in plain terms
A US size 0 corresponds to an inside diameter of 11.63 mm, and every full US size adds 0.8128 mm of diameter. The British scale starts at A and advances in roughly half-US-size steps, so one US size covers about two British letters.
Useful anchor points:
- US 5 = UK J½
- US 6 = UK L½
- US 7 = UK N½
- US 8 = UK P½
- US 9 = UK R½
- US 10 = UK T½
Notice that whole US sizes tend to land on British half letters. That is the single most common source of confusion when British customers buy from American retailers.
Why the drift happens
The American scale is arithmetic: fixed millimetre increments per size. The British scale was standardised separately by the British Standards Institution and its letters do not correspond to a round millimetre value. Where the two grids fall between each other, retailers round in different directions, so a US 7 might be listed as UK N by one shop and N½ by another.
Practical rules for buying abroad
Ignore the letter or number and work from millimetres wherever the retailer publishes them. Most reputable jewellers list inside diameter or circumference in the product specification. Matching the millimetre figure removes every rounding disagreement.
If only a size code is given, round in the direction of comfort. For a plain band, rounding up by a quarter size is safer than rounding down, because a ring that is slightly loose can be padded or resized down cheaply, while a ring that will not pass the knuckle is unwearable until it is stretched.
Half sizes and letters
Some British jewellers use half letters (N½) and some use plus notation (N+). They mean the same thing. If a British size is quoted without a half marker on a ring bought from a US listing, ask the seller for the millimetre measurement before ordering.
Quick check before you buy
Convert your size in both directions. Enter your UK letter into the converter, note the US number, then re-enter that US number and confirm it returns the letter you started with. If it does not, you are sitting on a rounding boundary and should choose based on the millimetre value instead.