A customer sends over a US size copied from an old ring, asks for the UK equivalent, and orders. It doesn’t fit. This happens more often than it should, and the reason isn’t carelessness. The two systems don’t share a common unit, so no conversion chart between them was ever going to be exact.
- US ring sizes are numbers and UK ring sizes are letters. Neither is a measurement you can check yourself with a ruler.
- Two conversion charts can disagree by half a size, because rounding happens twice: once turning a circumference into a US number, once turning it into a UK letter.
- Band width changes fit. A wide flat band feels tighter than a thin band cut to the identical size.
- A physical, graduated ring sizer beats any online chart, converted or not.
Two systems measuring the same thing, separately
US sizing runs numeric, in small fractions, starting around size 3 for a child’s finger and rising past 13 for a large hand. UK sizing runs alphabetic, A through Z, then Z+1, Z+2 and upward for anything larger. Both describe the same physical fact underneath: the inside circumference of the ring. But each was standardised on its own, on its own increments, and neither system prints that circumference on the ring itself. Converting between them means passing through a measurement that isn’t written down anywhere on the piece. Every extra conversion step is somewhere for rounding to drift, and that is exactly why two charts, each correct on its own terms, can land on different answers for the same finger.
Why the fit still moves once the number is right
A correctly converted size still doesn’t guarantee a correct fit, because a size chart describes circumference, not the finished ring. A wide, flat band sits against more of the finger than a thin, rounded band cut to the identical nominal size, so the wide one always feels tighter. Knuckle size, temperature and time of day move the true reading too, and none of that shows up on any chart.
At Silux London, every made-to-order piece is sized from a physical gauge reading, taken on the day, never from a number carried over from another ring or another country. A half size wrong is a remake, not a quick adjustment. If you’re ordering from a distance and can’t be measured in person, ask the maker for a printable or postable sizer rather than trusting a single chart. It costs a few days. A ring that has to go back for resizing costs more than that.